Sugarbush has a nice compromise between group and private - "max 4" lessons, small groups reasonable price, and they have one geared towards intermediates looking to start skiing advanced terrain:
Castlerock it's true has no easy way down, but Middle Earth & Castlerock run are only "advanced" - middle earth being steeper bump pitches with flat between, Castletock being a more even not too steep bump run. They used to sometime run a bump clinic on the Mt Ellen side, which is worth doing if they have it (I don't see it on the website, but you can ask).
Mad River has plenty of intermediate terrain, but not sure it is worth going there if you can't shred the gnar. Maybe on a weekday, if you don't mind riding up alone on the single.
If you like woods, start at Eden woods on sugarbush, then try deeper sleeper. On the Ellen side try semi-tough woods. Most of the other woods are steeper and tighter. Moonshine and Lixie's Twist are natural snow trails that should be challenging but fun for intermediates: bumped up, maybe icy, but not steep at all.
Have fun, the 'bush is a great resort!