So people keep saying to use manyus for leveling but my units that are level 30 and above receive less exp now, should I keep doing this or am I wasting the manyus effectiveness?

You should use the crappy manyu for 1-30, bronze for 30-50 and gold/bronze for anything 50+. If possible try to make sure you matching the types and always do 10 at a time. The ranges are just my personal preference but you can start using bronzes at 25 since the bad ones start falling off around there.