Quote:
it took too long when I wanted to change anything and remake.
That's because the memory size of your image was greater than the amount of ram in your computer.
When that happens Gimp uses the swap partition on your internal disk. That can be very slow.
If you are working on an image with enough ram then you can merge 2 animations into one and them merge that with another, and another etc.
Of course you work from the back (smallest) to the front or top layer.
Good on you for finding a work around