I second this. I would go for APNG instead GIF or WEBP/M :
- Transparency !
- Smaller in size
- Better global quality and restain all the color.
- That's the format used for Stickers in almost all messaging app (Line, Signal, Discord, etc)
- Easy to create, just use Apngasm
But not sure that Unity handle this format natively. Instead it could be easier to just import a bunch of frames for each image, as a sequence animation.