I wouldn't bother with drilled for the Auxiliary archers. In fact I'd field them in open order without the armour, and probably have one unit of 10 bases. That's 13 dice which is a very respectable amount to unload on a single target. With only 3 legionary formations, it might be wise to get a couple of minimum size units of Numeri skirmishers, which can hide behind the legions and then pop out to extend the Line of Battle when you need them. Finally with the Romans having the Stamina rule, you can get away with 5 bases in each unit.
"No one ever achieved anything without making a few mistakes along the way"