Okay your maneuvers (obviously) determine what you do. If you want to focus on offense and be primary damage dealer, that's a bit hard, but Stone Dragon, Desert Wind, maybe some Diamond Mind will do it for you. If you want to focus on mobility and winning, Diamond Mind, Setting Sun, Tiger Claw, and Shadow Hand are your best bet. You don't really need to multiclass to be effective.
Also your dm is making a dumb rule. Prestige classes aren't "new abilities" they're just abilities. Why is a SS20 any different from a SS10/Monk4/whatever? Explain to the DM that classes aren't like schools your character has to go to, you can just learn the abilities by fighting and shit.