A friend of mine who also wanted a crib that converts to a twin got a Natart crib. It was the only one she could find. Is there a reason you don't want that brand?
I agree with you about moving a child into a twin bid rather than a full. We used a standard crib. When DS moved into a twin bed, we found a nice headboard at a furniture clearance center and attached it to a regular mattress frame.
DS: age 9, my rare and extraordinary little man.
“Autistics are the ultimate square pegs, and the problem with pounding a square peg into a round hole is not that the hammering is hard work. It's that you're destroying the peg." - Paul Collins, Not Even Wrong