I actually prefer to have the Marian reforms be triggered by the player. The problem with mid-level units affects most of the civilized factions, not just the Roman ones. Changing the date of the Marian reforms won't fix this problem. The problem for me is that cities grow too slowly in the beginning and too fast in the end. Maybe adjusting the earlier cities so we can get large city at around 8000 would give a better window for using mid-level units.