Ronix makes the Phoenix Project, a hybrid flex board aimed mostly at park riding. I rode it and I thought it was rubbish, but I'm really not a fan of the way Ronix boards ride in general. You might like it.
I tried the track system on a 2012 Slingshot. The bolts slid to the end of the track no matter how hard I tightened them. Others seemed to have the same problem:
That said, Slingshots are super fun at the park. I recently bought a 2010 Recoil. It's super soft and flexy, it has a big 3-stage rocker, and it's really loose on the water, which I like. It doesn't have the fast track. You might want to look at 2010 Slingshot stuff because it's extremely cheap and I really didn't notice much difference between my 2010 and the 2012 I rode.