So, at last, some degree of success...

First things first: Before, I used imagemagick's "convert" to resize the PNG images eported from xsheet-mypaint. For clarity I did this on my laptop because of the ffmpeg version.

Two things here: A: The background layer, not part of the animation frames but used as the static background there, has different size dimensions than the frames themselves. B: After converting, some of the images changed from DirectClass to PseudoClass (checked it with "identify").

So I decided to take the exported images as they were and work with them, rather than resizing, etc.

Than I used:

Because of the smaller static background image, I had to move some items to the right and up, done by "overlay=".

Original dimensions of the background were 1042x1152 px and the animationframes were all 1536x1728 px.

Zipped result included in the attachment.