Fix: Run the FileAccess.get_file_as_bytes() method in a thread. I thought a thread wouldn't allow for loading files using FileAccess, but it seems I was mistaken. I personally just make the Thread run my _threaded_loading(file_path:String) method and then call a signal named loaded_bytes(data:PackedByteArray). The _threaded_loading method looks like this:

func _threaded_loading(path:String) -> void:
	print("I am the thread and I will now load this file")
	var file:FileAccess = FileAccess.open(path, FileAccess.READ)
	print("i have opened the file")
	var bytes:PackedByteArray = file.get_buffer(file.get_length())
	print("I have the bytes of the file")
	file.close()
	print("I have closed the file")
	call_deferred_thread_group("emit_signal", "loaded_bytes", bytes)
	return

It is important to emit the signal deferred, as a Thread on its own can't emit a signal on its own. I did print a lot in the method just to check that everything is working as intended. I hope this helps someone!

I have found that this is incredibly fast. Loading files in threads in insane. Here is a demo where Godot loads 456 files, which all together add up to 1.6 GB in size in less than four seconds https://files.catbox.moe/g28op4.webm

Have you tried asking this question in the Godot discord? The engine devs hang out there, might be able to give you a hand.