The Collings MT is a fine instrument, and you will have to look hard to find an upgrade from the MT to something with a scroll sonically. I think the next step up would be someting like a Gibson Sam Bush, but that is more money than you mention. You could go with an MF, but you would have two Collings then, which is in no way a bad thing. I wouldn't sell the MT to buy something else, because after a time the MT will have played in and it probably sounds pretty good by now. If you bought a new F-style instrument, you might be discouraged by the sound until it opens up. (When I bought my F-style mandolin, I drove the sellers nuts by making the ship me instruments just to ship them back...)
I should add, I own a Collings MF, I love it, and I would never sell it. But I have also played MTs quite a bit.
Having said that, I played a very nice used Flatiron F for sale the other day that is in your price range and sounded spectacular. Signed by Bruce Weber. You might check the classifieds for something like that.
At the risk of making trouble, it is hard to imagine a new $3500 F style mandolin sounding as good as a played in MT, but $3500 will buy a killer A syle oval. You already have a great bluegrass instrument in the MT, and you might want to consider an oval holed instrument as a step up instrument. Whatever you do, don't take any advice from me or anyone else other than this: play it before you buy it. Take your time. I have been planning my next acquisition for over a year now, and the longer I take, the more fun I have.
When I was trying to move out of the beginner zone (waidaminnut..I still am trying to move out of the beginner zone!) my wife sprang for some lessons. If you aren't taking lessons, the amount of money you would have spent on the upgrade but spent on lessons will make that MT sound mighty nice...assuming lessons work better for you than they did for me. Lessons won't do anything for MAS, though.
Ha, ha! keep time: how sour sweet music is,
When time is broke and no proportion kept!