diff --git a/Marlin/src/gcode/parser.cpp b/Marlin/src/gcode/parser.cpp
index 896ff273cd..bfa4346f03 100644
--- a/Marlin/src/gcode/parser.cpp
+++ b/Marlin/src/gcode/parser.cpp
@@ -150,12 +150,30 @@ void GCodeParser::parse(char *p) {
   #endif
 
   /**
-   * Screen for good command letters. G, M, and T are always accepted.
-   * With Motion Modes enabled any axis letter can come first.
+   * Screen for good command letters.
    * With Realtime Reporting, commands S000, P000, and R000 are allowed.
    */
+  #if ENABLED(REALTIME_REPORTING_COMMANDS)
+    switch (letter) {
+      case 'P': case 'R' ... 'S': {
+        uint8_t digits = 0;
+        char *a = p;
+        while (*a++ == '0') digits++; // Count up '0' characters
+        if (digits == 3) {            // Three '0' digits is a good command
+          codenum = 0;
+          command_letter = letter;
+          return;
+        }
+      }
+    }
+  #endif
+
+  /**
+   * Screen for good command letters. G, M, and T are always accepted.
+   * With Motion Modes enabled any axis letter can come first.
+   */
   switch (letter) {
-    case 'G': case 'M': case 'T': TERN_(MARLIN_DEV_MODE, case 'D':)
+    case 'G': case 'M': case 'T': TERN_(MARLIN_DEV_MODE, case 'D':) {
       // Skip spaces to get the numeric part
       while (*p == ' ') p++;
 
@@ -177,20 +195,18 @@ void GCodeParser::parse(char *p) {
       // A '?' signifies an unknown command
       command_letter = letter;
 
-      {
-        #if ENABLED(SIGNED_CODENUM)
-          int sign = 1; // Allow for a negative code like D-1 or T-1
-          if (*p == '-') { sign = -1; ++p; }
-        #endif
+      #if ENABLED(SIGNED_CODENUM)
+        int sign = 1; // Allow for a negative code like D-1 or T-1
+        if (*p == '-') { sign = -1; ++p; }
+      #endif
 
-        // Get the code number - integer digits only
-        codenum = 0;
+      // Get the code number - integer digits only
+      codenum = 0;
 
-        do { codenum = codenum * 10 + *p++ - '0'; } while (NUMERIC(*p));
+      do { codenum = codenum * 10 + *p++ - '0'; } while (NUMERIC(*p));
 
-        // Apply the sign, if any
-        TERN_(SIGNED_CODENUM, codenum *= sign);
-      }
+      // Apply the sign, if any
+      TERN_(SIGNED_CODENUM, codenum *= sign);
 
       // Allow for decimal point in command
       #if USE_GCODE_SUBCODES
@@ -213,38 +229,33 @@ void GCodeParser::parse(char *p) {
         }
       #endif
 
-      break;
+      } break;
 
     #if ENABLED(GCODE_MOTION_MODES)
-      case 'I' ... 'J':
-        if (motion_mode_codenum != 5 && \
-            TERN1(ARC_SUPPORT, motion_mode_codenum != 2 && motion_mode_codenum != 3)) return;
-      case 'Q':
-        if (motion_mode_codenum != 5) return;
+
+      #if EITHER(BEZIER_CURVE_SUPPORT, ARC_SUPPORT)
+        case 'I' ... 'J': case 'P':
+          if (TERN1(BEZIER_CURVE_SUPPORT, motion_mode_codenum != 5)
+            && TERN1(ARC_P_CIRCLES, !WITHIN(motion_mode_codenum, 2, 3))
+          ) return;
+      #endif
+
+      #if ENABLED(BEZIER_CURVE_SUPPORT)
+        case 'Q': if (motion_mode_codenum != 5) return;
+      #endif
+
+      #if ENABLED(ARC_SUPPORT)
+        case 'R': if (!WITHIN(motion_mode_codenum, 2, 3)) return;
+      #endif
+
       case 'X' ... 'Z': case 'E' ... 'F':
         if (motion_mode_codenum < 0) return;
         command_letter = 'G';
         codenum = motion_mode_codenum;
         TERN_(USE_GCODE_SUBCODES, subcode = motion_mode_subcode);
         p--; // Back up one character to use the current parameter
-      break;
-    #endif
+        break;
 
-    #if ENABLED(REALTIME_REPORTING_COMMANDS)
-      case 'P': case 'R': {
-        if (letter == 'R') {
-          #if ENABLED(GCODE_MOTION_MODES)
-            if (ENABLED(ARC_SUPPORT) && !WITHIN(motion_mode_codenum, 2, 3)) return;
-          #endif
-        }
-        else if (TERN0(GCODE_MOTION_MODES, motion_mode_codenum != 5)) return;
-      } // fall-thru
-      case 'S': {
-        codenum = 0;                  // The only valid codenum is 0
-        uint8_t digits = 0;
-        while (*p++ == '0') digits++; // Count up '0' characters
-        command_letter = (digits == 3) ? letter : '?'; // Three '0' digits is a good command
-      } return;                       // No parameters needed, so return now
     #endif
 
     default: return;
@@ -252,18 +263,12 @@ void GCodeParser::parse(char *p) {
 
   // The command parameters (if any) start here, for sure!
 
-  #if DISABLED(FASTER_GCODE_PARSER)
-    command_args = p; // Scan for parameters in seen()
-  #endif
+  IF_DISABLED(FASTER_GCODE_PARSER, command_args = p); // Scan for parameters in seen()
 
   // Only use string_arg for these M codes
   if (letter == 'M') switch (codenum) {
-    #if ENABLED(GCODE_MACROS)
-      case 810 ... 819:
-    #endif
-    #if ENABLED(EXPECTED_PRINTER_CHECK)
-      case 16:
-    #endif
+    TERN_(GCODE_MACROS, case 810 ... 819:)
+    TERN_(EXPECTED_PRINTER_CHECK, case 16:)
     case 23: case 28: case 30: case 117 ... 118: case 928:
       string_arg = unescape_string(p);
       return;