In case it helps anyone... this is how I've implemented the workaround from the GitHub issue page. It lets me add new Animation Mixes with a single click without crashing Godot. This is on my custom SpineManager script that inherits from SpineSprite. The script must be a tool script for this to work. To add an Animation Mix, I just click the "Add Anim Mix Entry" bool in the Inspector.
@tool
class_name SpineManager
extends SpineSprite
@export var add_anim_mix_entry: bool = false:
set(value):
if add_anim_mix_entry == value: return
add_animation_mix_entry()
func add_animation_mix_entry() -> void:
#This exists as a temporary workaround for the Spine GDExtension glitch that
#causes Godot to crash when trying to add a new Animation Mix entry in the
#inspector. Can be removed once Esoteric fixes this glitch.
if !Engine.is_editor_hint(): return
var skeleton_file_res: SpineSkeletonFileResource = skeleton_data_res.skeleton_file_res
skeleton_data_res.skeleton_file_res = null
skeleton_data_res.animation_mixes.append(SpineAnimationMix.new())
skeleton_data_res.skeleton_file_res = skeleton_file_res
