My animations work great on my inital model but transfering it to my other model (same model) breaks it

there is no reason to save the animations as resources.
maybe to add custom keys, but that is a lot of work so should only be done when it’s needed.
In my case I run a script on the AnimationTree playback to check the name of the current animation and the time and execute code between a two times that match the animation. for example, when the attack animation connects, I go out of the attack animation, or deal damage, or play a sound.
that way I don’t have to save and edit an animation.

@onready var state : AnimationNodeStateMachinePlayback = get("parameters/Movement/playback")
var ppos : float = 0.0

func _physics_process(delta) -> void:
	ppos = state.get_current_play_position()
	match state.get_current_node():
		"Axe":
			canJump = true
		"Walk":
			canJump = true
		"Freelook":
			canJump = true
		"Melee Attack Horizontal":
			if ppos > 0.6 and ppos < 0.7:
				Attack()

yes. you don’t need skins.
you only need skins if you are exporting the file in fbx, because unused bones are deleted. that is not a problem with glb.
download the fbx from mixamo without skin, import them in blender and export a bunch of animations to a single glb file.

that is the correct way to do it, the last part “separated it later” is confusing me a bit.
if you need the animations in different files, import and export them to different files. for this, you save the fbx to different folders, work on different blends, and export to different glbs.

mixamo animations tend to be in T-POSE. if the animations where it the same pose as your model, there would be no problem.
you have two options:
1 - edit the skeleton in blender to make it T-POSE. this could have some problems because it would not be perfectly aligned and there are differences in bone rotation and roll, and the positions of the legs, but it would make it look much better.
2 - load your model into mixamo and then download it (with skin), this will convert it to T-POSE.
before doing that you need to rename all the bones to mixamorig:etc, like mixamorig:chest or mixamorig:LeftArm and make sure the bones are the same and there is no Root bone. the first bone should be Hips.
after this, if there are no big name and skeleton differences, the animations should play correctly.

here’s the workflow from mixamo to godot, it is very similar to what you would find in some tutorials: