A good way to handle keeping your armies updated / full strength while migrating your troops back to places where they can be trained is to leave a small ready-reserve outside the walls of the town outside of the AI's usual path (I usually build a fort in these instances) where you can bring your units back to full strength and ship the smaller units back to your homeland for re-training. Costs a bit more but is functional.

After the Marian reforms you can recruit your legions pretty much everywhere where Rome historically conquered once building the barracks (and get some nice locale-specific Auxilia as well.)

For some of the other factions with weaker units, I might drop a client kingdom in a central area if I want access to, say, successor style units that can be recruited locally.

This is one of the aspects I personally like most about the way EB handles recruitment for pretty much anyone but Post Marian Romans (or Greeks who seem to be all over the darned place )