It seems like you're overanalyzing this. More volume = more air for vertical moves, but it can be harder for beginners to get the ends down in order to begin the moves. Just check out some videos on playboating, as well as try putting some water in the boat, which both should help. Also, since you said that you had to take out the hip pads to be comfortable, you might want to check out a Jackson, which are the only boats that I know of that have progressively larger cockpits for each larger model. If you want to do more playboating, don't get a boat like the Axiom, since it is not designed as a modern playboat. It may work for enders, cartwheels, and pivot turns, but that's about it.
You said that some boats are painful to sit in, but if they are outfitted properly, then if you fit inside of the boat with a little extra foot room (1-2" min.) then you should be able to put in some foam to cushion your feet, thigh risers and knee blocks to lock your legs in the thigh braces, and that should get rid of most of the pain. The thigh risers are amazing; I had pain whenever I would roll/attempt to roll, and now I am really comfortable in my boat, with no problems whatsoever.
If you want a river runner, I would go with a higher-volume boat like a Liquid Logic Remix, Pyranha Karnali/Burn, etc. that will have a bit more speed than a playboat, which should help in punching holes. If you really want a river-running playboat, go with a Pyranha Recoil or JK Fun, since they aren't much longer than regular playboats.