As I already wrote it in my first post, you need to ensure clear, unambiguous edges.
And, you can already pay attention to this when creating the images. If, as in this picture, you want to have a blending effect or smearing and smudging effect on some transitions/edges, then you could first paint over the edges a little and then “cut off” the painted-over area by erasing it from the outside with a brush with defined edges.
I would also definitely work with layers, you can merge them again at the end, and also in between, if there seem to be too many layers and they become confusing. In between, it may make sense to merge two or three layers occasionally, but I always try to avoid this as long as possible.
This, in combination with @Deif_Lou’s hints when/where needed, should give you what you want.
Michelist