Usually people that are trying to learn and want to start understanding rsps, usually go with something closer to base and doesn't have a lot of content.
I personally started with vencillio and went on to ruse/elvarg because they are the easiest to understand and didn't have that much content involved. I personally don't like PI because it's messy and probably the furthest away from java standards than the other two, if I would give a specific suggestion I would try Elvarg and see what you can do with that.