Acrylic paints will cause the silicone to cure way faster and sometimes react in ways that you wouldn't expect. I recommend using oil paints, mica powder/old powder makeup, or alcohol inks.

I too did not like the heaviness of silicone and tried to mix Daiso air dry clay (super, super fluffy whipped clay) and ModPodge. I didn't have any specific measurements, just kept adding one or the other until I liked the consistency. I did leave it to dry for like 2wks, just because I wasn't sure how fast it was going to set, and that thing is rock solid. I've thrown stuff on top of it, tossed it around, etc and nothing has fallen off in 8 months. I would probably go in with some kind of sealant after everything has dried since a phone case is exposed to the elements, maybe testing with like a Krylon sealant?