For reference I've never played Darkest Dungeon or any of its clones so if a lot of my complaints are established conventions of the genre you can probably safely ignore them. Incoming spam:
Not a fan of the grain effect in battle. The animation is just horizontal flipping. The layering of it could be better I think. It's hard to explain but since the backgrounds have a bit of perspective (so you can tell some parts of the ground are suppose to be "in front" of the player), the way the grain effect is layered makes the scene look more 2D than I think it should. I get why you have it like this, to keep important stuff less obscured but I think you should either have the grain effect only selectively cover things, (i.e. only the trees behind the characters in the forest background) or have it cover everything but with a reduced opacity. Also you should add more distinct frames for the grain animation to make it look more like an old film reel.
If the last enemy is killed early in the turn, remaining characters still perform queued moves on nothing. Battle should probably end immediately after the last enemy dies.
There's no way to cancel a move selection using "BACK". Since moves execute only after all selections, you should let the player be able to cancel earlier moves to correct misinputs or if they change their mind.
I noticed the knight's spear sometimes obscures the action select menu. You should update your code to ensure ui elements always render on top. In Godot, CanvasLayer is really good for this if you're not using it already.
After selecting a move for one character, the cursor should auto-move to the next character who hasn’t acted yet.
You probably already know this but Rogue and Cleric portraits are the same in the character/details menu.
I think the turn order display is unclear. If its pre-determined by speed, maybe just display the full turn-order at the start of the player's turn so they can strategize around it instead of having it gradually display as each selection is made.
Pressing BACK on the party menu returns to the title screen instead of the battle / progress menu.
Pressing BACK from the equipment menu goes to the character/details menu, not directly to the party menu.
There's a minor ui inconsistency in the character/details menu. The bottom-right label says "Character" but the button that takes you there is called "Details". I feel these should both be named the same thing.
During the last battle I noticed character sprites were being positioned strangely around the place during their attack animations. Like the boss would suddenly appear behind the knight as they do their attack animation.
Anyways that's all I noticed while playing. The sprites and artwork are gorgeous btw. Very nice stuff.