diff --git a/Marlin/src/HAL/AVR/HAL.h b/Marlin/src/HAL/AVR/HAL.h
index 7185142175..1491867721 100644
--- a/Marlin/src/HAL/AVR/HAL.h
+++ b/Marlin/src/HAL/AVR/HAL.h
@@ -19,6 +19,10 @@
  */
 #pragma once
 
+/**
+ * HAL for Arduino AVR
+ */
+
 #include "../shared/Marduino.h"
 #include "../shared/HAL_SPI.h"
 #include "fastio.h"
diff --git a/Marlin/src/core/serial_hook.h b/Marlin/src/core/serial_hook.h
index 3972279452..65c553c702 100644
--- a/Marlin/src/core/serial_hook.h
+++ b/Marlin/src/core/serial_hook.h
@@ -43,7 +43,9 @@ public:
   }
 
   constexpr SerialMask(const uint8_t mask) : mask(mask) {}
-  constexpr SerialMask(const SerialMask & other) : mask(other.mask) {} // Can't use = default here since not all framework support this
+  constexpr SerialMask(const SerialMask &rs) : mask(rs.mask) {} // Can't use = default here since not all frameworks support this
+
+  SerialMask& operator=(const SerialMask &rs) { mask = rs.mask; return *this; }
 
   static constexpr uint8_t All = 0xFF;
 };
diff --git a/Marlin/src/core/types.h b/Marlin/src/core/types.h
index 1e80f8059c..dace435fd1 100644
--- a/Marlin/src/core/types.h
+++ b/Marlin/src/core/types.h
@@ -91,9 +91,9 @@ struct Flags {
   void set(const int n)                    { b |=  (bits_t)_BV(n); }
   void clear(const int n)                  { b &= ~(bits_t)_BV(n); }
   bool test(const int n) const             { return TEST(b, n); }
-        bool operator[](const int n)       { return test(n); }
-  const bool operator[](const int n) const { return test(n); }
-  const int size() const                   { return sizeof(b); }
+  bool operator[](const int n)             { return test(n); }
+  bool operator[](const int n) const       { return test(n); }
+  int size() const                         { return sizeof(b); }
 };
 
 // Specialization for a single bool flag
@@ -105,9 +105,9 @@ struct Flags<1> {
   void set(const int)                     { b = true; }
   void clear(const int)                   { b = false; }
   bool test(const int) const              { return b; }
-        bool operator[](const int)        { return b; }
-  const bool operator[](const int) const  { return b; }
-  const int size() const                  { return sizeof(b); }
+  bool operator[](const int)              { return b; }
+  bool operator[](const int) const        { return b; }
+  int size() const                        { return sizeof(b); }
 };
 
 typedef Flags<8> flags_8_t;
@@ -124,9 +124,9 @@ typedef struct AxisFlags {
   void set(const int n, const bool onoff)  { flags.set(n, onoff); }
   void clear(const int n)                  { flags.clear(n); }
   bool test(const int n) const             { return flags.test(n); }
-        bool operator[](const int n)       { return flags[n]; }
-  const bool operator[](const int n) const { return flags[n]; }
-  const int size() const                   { return sizeof(flags); }
+  bool operator[](const int n)             { return flags[n]; }
+  bool operator[](const int n) const       { return flags[n]; }
+  int size() const                         { return sizeof(flags); }
 } axis_flags_t;
 
 //
diff --git a/Marlin/src/gcode/config/M200-M205.cpp b/Marlin/src/gcode/config/M200-M205.cpp
index 8b50437eef..82a110e06b 100644
--- a/Marlin/src/gcode/config/M200-M205.cpp
+++ b/Marlin/src/gcode/config/M200-M205.cpp
@@ -135,8 +135,8 @@ void GcodeSuite::M201() {
 
   LOOP_LOGICAL_AXES(i) {
     if (parser.seenval(AXIS_CHAR(i))) {
-      const uint8_t a = TERN(HAS_EXTRUDERS, (i == E_AXIS ? uint8_t(E_AXIS_N(target_extruder)) : i), i);
-      planner.set_max_acceleration(a, parser.value_axis_units((AxisEnum)a));
+      const AxisEnum a = TERN(HAS_EXTRUDERS, (i == E_AXIS ? E_AXIS_N(target_extruder) : (AxisEnum)i), (AxisEnum)i);
+      planner.set_max_acceleration(a, parser.value_axis_units(a));
     }
   }
 }
@@ -184,8 +184,8 @@ void GcodeSuite::M203() {
 
   LOOP_LOGICAL_AXES(i)
     if (parser.seenval(AXIS_CHAR(i))) {
-      const uint8_t a = TERN(HAS_EXTRUDERS, (i == E_AXIS ? uint8_t(E_AXIS_N(target_extruder)) : i), i);
-      planner.set_max_feedrate(a, parser.value_axis_units((AxisEnum)a));
+      const AxisEnum a = TERN(HAS_EXTRUDERS, (i == E_AXIS ? E_AXIS_N(target_extruder) : (AxisEnum)i), (AxisEnum)i);
+      planner.set_max_feedrate(a, parser.value_axis_units(a));
     }
 }
 
diff --git a/Marlin/src/lcd/extui/ui_api.cpp b/Marlin/src/lcd/extui/ui_api.cpp
index 7792acd491..143f240c2d 100644
--- a/Marlin/src/lcd/extui/ui_api.cpp
+++ b/Marlin/src/lcd/extui/ui_api.cpp
@@ -656,7 +656,7 @@ namespace ExtUI {
   }
 
   void setAxisMaxFeedrate_mm_s(const feedRate_t value, const axis_t axis) {
-    planner.set_max_feedrate(axis, value);
+    planner.set_max_feedrate((AxisEnum)axis, value);
   }
 
   void setAxisMaxFeedrate_mm_s(const feedRate_t value, const extruder_t extruder) {
@@ -674,7 +674,7 @@ namespace ExtUI {
   }
 
   void setAxisMaxAcceleration_mm_s2(const_float_t value, const axis_t axis) {
-    planner.set_max_acceleration(axis, value);
+    planner.set_max_acceleration((AxisEnum)axis, value);
   }
 
   void setAxisMaxAcceleration_mm_s2(const_float_t value, const extruder_t extruder) {
diff --git a/Marlin/src/module/motion.cpp b/Marlin/src/module/motion.cpp
index a64f426fba..18298654f5 100644
--- a/Marlin/src/module/motion.cpp
+++ b/Marlin/src/module/motion.cpp
@@ -1399,7 +1399,7 @@ void prepare_line_to_destination() {
   bool homing_needed_error(linear_axis_bits_t axis_bits/*=linear_bits*/) {
     if ((axis_bits = axes_should_home(axis_bits))) {
       PGM_P home_first = GET_TEXT(MSG_HOME_FIRST);
-      char msg[strlen_P(home_first)+1];
+      char msg[30];
       sprintf_P(msg, home_first,
         NUM_AXIS_LIST(
           TEST(axis_bits, X_AXIS) ? STR_A : "",
diff --git a/Marlin/src/module/planner.cpp b/Marlin/src/module/planner.cpp
index 33c8913639..6aa363890e 100644
--- a/Marlin/src/module/planner.cpp
+++ b/Marlin/src/module/planner.cpp
@@ -3292,7 +3292,7 @@ void Planner::refresh_positioning() {
 }
 
 // Apply limits to a variable and give a warning if the value was out of range
-inline void limit_and_warn(float &val, const uint8_t axis, PGM_P const setting_name, const xyze_float_t &max_limit) {
+inline void limit_and_warn(float &val, const AxisEnum axis, PGM_P const setting_name, const xyze_float_t &max_limit) {
   const uint8_t lim_axis = TERN_(HAS_EXTRUDERS, axis > E_AXIS ? E_AXIS :) axis;
   const float before = val;
   LIMIT(val, 0.1, max_limit[lim_axis]);
@@ -3311,7 +3311,7 @@ inline void limit_and_warn(float &val, const uint8_t axis, PGM_P const setting_n
  *
  * This hard limit is applied as a block is being added to the planner queue.
  */
-void Planner::set_max_acceleration(const uint8_t axis, float inMaxAccelMMS2) {
+void Planner::set_max_acceleration(const AxisEnum axis, float inMaxAccelMMS2) {
   #if ENABLED(LIMITED_MAX_ACCEL_EDITING)
     #ifdef MAX_ACCEL_EDIT_VALUES
       constexpr xyze_float_t max_accel_edit = MAX_ACCEL_EDIT_VALUES;
@@ -3334,7 +3334,7 @@ void Planner::set_max_acceleration(const uint8_t axis, float inMaxAccelMMS2) {
  *
  * This hard limit is applied as a block is being added to the planner queue.
  */
-void Planner::set_max_feedrate(const uint8_t axis, float inMaxFeedrateMMS) {
+void Planner::set_max_feedrate(const AxisEnum axis, float inMaxFeedrateMMS) {
   #if ENABLED(LIMITED_MAX_FR_EDITING)
     #ifdef MAX_FEEDRATE_EDIT_VALUES
       constexpr xyze_float_t max_fr_edit = MAX_FEEDRATE_EDIT_VALUES;
diff --git a/Marlin/src/module/planner.h b/Marlin/src/module/planner.h
index a5c3bf51a4..bef381b5c7 100644
--- a/Marlin/src/module/planner.h
+++ b/Marlin/src/module/planner.h
@@ -501,10 +501,10 @@ class Planner {
     static void refresh_positioning();
 
     // For an axis set the Maximum Acceleration in mm/s^2
-    static void set_max_acceleration(const uint8_t axis, float inMaxAccelMMS2);
+    static void set_max_acceleration(const AxisEnum axis, float inMaxAccelMMS2);
 
     // For an axis set the Maximum Feedrate in mm/s
-    static void set_max_feedrate(const uint8_t axis, float inMaxFeedrateMMS);
+    static void set_max_feedrate(const AxisEnum axis, float inMaxFeedrateMMS);
 
     // For an axis set the Maximum Jerk (instant change) in mm/s
     #if HAS_CLASSIC_JERK
diff --git a/Marlin/src/module/temperature.cpp b/Marlin/src/module/temperature.cpp
index f65de00f61..531c52c09d 100644
--- a/Marlin/src/module/temperature.cpp
+++ b/Marlin/src/module/temperature.cpp
@@ -751,7 +751,7 @@ volatile bool Temperature::raw_temps_ready = false;
       // Report heater states every 2 seconds
       if (ELAPSED(ms, next_temp_ms)) {
         #if HAS_TEMP_SENSOR
-          print_heater_states(ischamber ? active_extruder : (isbed ? active_extruder : heater_id));
+          print_heater_states(heater_id < 0 ? active_extruder : (int8_t)heater_id);
           SERIAL_EOL();
         #endif
         next_temp_ms = ms + 2000UL;
@@ -2817,6 +2817,9 @@ void Temperature::init() {
 
 #if HAS_THERMAL_PROTECTION
 
+  #pragma GCC diagnostic push
+  #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+
   Temperature::tr_state_machine_t Temperature::tr_state_machine[NR_HEATER_RUNAWAY]; // = { { TRInactive, 0 } };
 
   /**
@@ -2946,6 +2949,8 @@ void Temperature::init() {
     }
   }
 
+  #pragma GCC diagnostic pop
+
 #endif // HAS_THERMAL_PROTECTION
 
 void Temperature::disable_all_heaters() {
@@ -3669,6 +3674,9 @@ void Temperature::isr() {
 
   switch (adc_sensor_state) {
 
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+
     case SensorsReady: {
       // All sensors have been read. Stay in this state for a few
       // ISRs to save on calls to temp update/checking code below.
@@ -3686,6 +3694,8 @@ void Temperature::isr() {
       }
     }
 
+    #pragma GCC diagnostic pop
+
     case StartSampling:                                   // Start of sampling loops. Do updates/checks.
       if (++temp_count >= OVERSAMPLENR) {                 // 10 * 16 * 1/(16000000/64/256)  = 164ms.
         temp_count = 0;
@@ -3917,7 +3927,7 @@ void Temperature::isr() {
     delay(2);
   }
 
-  void Temperature::print_heater_states(const uint8_t target_extruder
+  void Temperature::print_heater_states(const int8_t target_extruder
     OPTARG(HAS_TEMP_REDUNDANT, const bool include_r/*=false*/)
   ) {
     #if HAS_TEMP_HOTEND
diff --git a/Marlin/src/module/temperature.h b/Marlin/src/module/temperature.h
index 9dc41129b2..da9d4a071b 100644
--- a/Marlin/src/module/temperature.h
+++ b/Marlin/src/module/temperature.h
@@ -995,7 +995,7 @@ class Temperature {
     #endif // HEATER_IDLE_HANDLER
 
     #if HAS_TEMP_SENSOR
-      static void print_heater_states(const uint8_t target_extruder
+      static void print_heater_states(const int8_t target_extruder
         OPTARG(HAS_TEMP_REDUNDANT, const bool include_r=false)
       );
       #if ENABLED(AUTO_REPORT_TEMPERATURES)