There are very cheap decompression algorithms. Most importantly for textures...

The ENTITY struct isn't big. It takes 4KB or something like that. 100 skills or 20 make no difference.

Bone animation is expensive but there is no magic trick to make it faster. I don't know if it can be done on mobile GPUs. Probably the newer ones with shader support can accelerate it. Otherwise simply use much cheaper vertex animation instead.

You have to live with the fact that your games have to be much simpler but this isn't really an engine thing. The engine would need to support OpenGL ES and Lite-C ARM processors but not much else would have to change.