Sounds like you still need to learn to use rudder - it's what keeps you from losing altitude in slower rolls, and obviously what you need for knife edge. The 4 Star with the AX is very capable - I flew the Dolphin which is a copy, with the AX. I flew it in a pattern meet, too. Check out the pattern or IMAC forums and get some info on how to PROPERLY fly aerobatic maneuvers. It will really challenge you and improve your flying, then you can make a sound decision on what plane to get next. Use lots of rudder - do flat turns, coordinated turns, stall turns. Use rudder to track straight through a loop so you don't get blown in or out. Keep practicing the knife edge. Probably need full throttle, and most to full rudder, and feathering the elevator. Learn to fly knife edge well on this plane and you will appreciate your next plane even more. I could fly the Dolphin in a knife edge around the pattern all day if I had to.

For the sharp moves and losing speed - what prop do you have? Try a 12x5, you will love it for the type of flying you're doing, granted your engine is fully broken in. If not, an 11x5.

And practice using your rudder

Nice job by the way! Keep it up.