Class design mockup. I think this will fix every issue with the current paint module. It's main downside will be that it will depend on the editor a lot more, so it will need to have more code if it needs to run in-app (not that much more code though, and mostly copy paste level stuff).

game/Node2D.tscn

@@ -1,3 +0,0 @@
-[gd_scene format=2]
-
-[node name="Node2D" type="Node2D"]

game/PaintCanvas.gd

@@ -0,0 +1,21 @@
+extends Node2D
+
+# Canvas that can be used to draw on
+
+# Should probably store it's actual data in an lz4 compressed poolvector property 
+# like what voxelman does
+
+# Undo history should be handled by the editor itself
+
+# It could use the paint module's actions for manipulation
+
+# it could have a drag and drop image property so things can be dropped into it
+
+# could be "infinite", so ti does not need to mess with resizes -> needs chunks though
+# if needed this should likely be an another class, like an InifinePaintCanvas or ChunkedPaintCanvas
+
+# better for now:
+# it could have it's own size -> when added to the tree it should copy the size of it's
+# parent paint project
+# then it should have a resize button with properties, ot it's sidebar module should have a button
+# + popup to handle resizes

game/PaintCurve.gd

@@ -0,0 +1,11 @@
+extends Path2D
+
+export(int, "append,union,sub,simdiff,etc...") var operation : int = 0
+
+# The renderer should process each child one by one in tree order
+# A render needs to start with a curve with an append operation
+# apply all non append curves to the last append curve
+# render it when out of children or when the next append curve is reached
+
+# It could just render only one append curves aswell, it could warn if you have more than one
+# This will need testing

game/PaintGroup.gd

@@ -0,0 +1,7 @@
+extends Node2D
+
+# Collects all child PaintNodes and renders them to it's texture
+# updates tex when a child changes
+# PaintNodes will need to signal their parents when they changed
+
+# update should run in process to save potential cycles

game/PaintNode.gd

@@ -0,0 +1,8 @@
+extends Node
+# PaintNode
+# base class for paint related classes
+
+# should inherit either Node2D or CanvasItem directly
+# Node2D is probably better
+
+# 

game/PaintProject.gd

@@ -0,0 +1,27 @@
+tool
+extends Node2D #PaintNode
+
+export(Vector2i) var image_size : Vector2i = Vector2i(64, 64)
+
+export(String) var image_file_name : String
+export(int, "png") var image_type : int
+
+# manages Grid, and BG's size
+# also lets you hide them, so custom ones can be created
+# could have an api to register backgorund layers and then it will resize them
+
+# Image size will only affect rendering at the end when saving the image
+
+func save_image() -> void:
+	pass
+
+func _get_property_list() -> Array:
+	return [
+		{
+			"type": TYPE_NIL,
+			"name": "save_image",
+			"hint": PROPERTY_HINT_BUTTON,
+			"hint_string": "save_image:Save/EditorIcons"
+		}
+		
+	]

game/PaintProjectMockup.tscn

@@ -0,0 +1,96 @@
+[gd_scene load_steps=8 format=2]
+
+[ext_resource path="res://PaintProject.gd" type="Script" id=1]
+[ext_resource path="res://PaintCanvas.gd" type="Script" id=2]
+[ext_resource path="res://PaintRenderer.gd" type="Script" id=3]
+[ext_resource path="res://PaintCurve.gd" type="Script" id=4]
+[ext_resource path="res://PaintGroup.gd" type="Script" id=5]
+[ext_resource path="res://Paintfilter.gd" type="Script" id=6]
+
+[sub_resource type="Curve2D" id=1]
+_data = {
+"points": PoolVector2Array( -126.479, 82.1171, 126.479, -82.1171, 239, 28, 0, 0, 0, 0, 461, 58, 0, 0, 0, 0, 491, 198, 0, 0, 0, 0, 494, 346, 0, 0, 0, 0, 107, 431, 0, 0, 0, 0, 43, 257, 0, 0, 0, 0, 201, 99, 0, 0, 0, 0, 242, 75, 0, 0, 0, 0, 214, 282, 0, 0, 0, 0, 329, 186, 0, 0, 0, 0, 368, 121, 0, 0, 0, 0, 338, 82, 0, 0, 0, 0, 284, 62, 0, 0, 0, 0, 239, 28 )
+}
+
+[node name="PaintProject (PaintNode)" type="Node2D"]
+script = ExtResource( 1 )
+save_image = null
+
+[node name="Grid (hidden)" type="Node" parent="."]
+
+[node name="BG (hidden)" type="Node" parent="."]
+
+[node name="PaintCanvas" type="Node2D" parent="."]
+script = ExtResource( 2 )
+
+[node name="PaintCanvas2" type="Node2D" parent="."]
+
+[node name="PaintGroup" type="Node2D" parent="."]
+
+[node name="PaintCanvas3" type="Node2D" parent="PaintGroup"]
+
+[node name="PaintCanvas4" type="Node2D" parent="PaintGroup"]
+
+[node name="PaintGroup" type="Node2D" parent="PaintGroup"]
+script = ExtResource( 5 )
+
+[node name="PaintCurveRender" type="Node2D" parent="PaintGroup/PaintGroup"]
+__meta__ = {
+"_editor_description_": "I'm hoping to use the built in svg renderer to rasterize curves. If it works, it will be a huge timesave. Should foreach children and apply them one by one based on their operation setting."
+}
+
+[node name="PaintCurve" type="Path2D" parent="PaintGroup/PaintGroup/PaintCurveRender"]
+curve = SubResource( 1 )
+script = ExtResource( 4 )
+__meta__ = {
+"_editor_description_": ""
+}
+
+[node name="PaintPathObject" type="Node2D" parent="PaintGroup/PaintGroup"]
+__meta__ = {
+"_editor_description_": "Maybe"
+}
+
+[node name="PaintCurveRender2" type="Node2D" parent="PaintGroup/PaintGroup"]
+
+[node name="PaintCurve" type="Path2D" parent="PaintGroup/PaintGroup/PaintCurveRender2"]
+curve = SubResource( 1 )
+script = ExtResource( 4 )
+__meta__ = {
+"_editor_description_": ""
+}
+
+[node name="PaintCurve1" type="Path2D" parent="PaintGroup/PaintGroup/PaintCurveRender2"]
+curve = SubResource( 1 )
+script = ExtResource( 4 )
+__meta__ = {
+"_editor_description_": ""
+}
+
+[node name="PaintCurve2" type="Path2D" parent="PaintGroup/PaintGroup/PaintCurveRender2"]
+curve = SubResource( 1 )
+script = ExtResource( 4 )
+__meta__ = {
+"_editor_description_": "Union'd with PaintCurve1."
+}
+operation = 1
+
+[node name="PaintGroup" type="Node2D" parent="PaintGroup/PaintGroup"]
+
+[node name="PaintCanvas6" type="Node2D" parent="PaintGroup/PaintGroup/PaintGroup"]
+
+[node name="PaintCanvas7" type="Node2D" parent="PaintGroup/PaintGroup/PaintGroup"]
+
+[node name="PaintCanvas7" type="Node2D" parent="PaintGroup"]
+
+[node name="PaintRenderer" type="Node2D" parent="PaintGroup"]
+script = ExtResource( 3 )
+
+[node name="PaintFilter" type="Node2D" parent="PaintGroup"]
+script = ExtResource( 6 )
+
+[node name="PaintCanvas8" type="Node2D" parent="PaintGroup/PaintFilter"]
+
+[node name="PaintCanvas9" type="Node2D" parent="PaintGroup/PaintFilter"]
+
+[node name="PaintCanvas10" type="Node2D" parent="PaintGroup/PaintFilter"]

game/PaintRenderer.gd

@@ -0,0 +1,9 @@
+extends Node2D
+
+# inherith from PaintNode
+
+# could be a thin base class for things that just render something
+# like noises etc
+# could have one for MMMaterial interp
+
+# over PaintNode they would just store and remember their parent PaintProject's size

game/Paintfilter.gd

@@ -0,0 +1,7 @@
+extends Node2D
+
+# inherits from PaintGroup
+# Same as PaintGroup except applies a filter to it's output
+# good for non destructive editing
+
+# Could also do resampling, resizing, shringking etc

game/editor.gd

@@ -0,0 +1,18 @@
+extends Node
+
+# Paint editor module
+
+# Sidebar
+
+# 1 class
+# by default it should have the color grid, and a color chooser, and module support
+# modules could be added for paintnode derived classes
+# it just forwards the currect selction to hem so they can
+# handle whether they want to be shown for a particular class
+
+# should have customizable location (left / right) Voxelman has the code
+
+# Other
+
+# the top file new etc menu is not needed, as now all of that are going to be handled via nodes and 
+# the editor's functionality