Removing ARR EX trials (HW and SB never pop as most find out about PF use by then) from the roulette would do more harm than good. Queue times would increase even further and you'd have almost no chance of replacing someone that left or was removed.

While doing the fights synced is better than unsynced, you're still not getting anything close to the original experience or difficulty. Ilvl increases, echo use and combat changes make them much easier.

DF is fine for the ARR EX trials provided the new players check out a guide before queueing and have some familiarity with how basic gameplay works.

Separating them into a roulette would open it up to queue manipulation during low-pop hours by groups only interested in getting the rewards with as little effort as possible.