About being steamrolled early, here's a notion:

Elite units get recruited in population cntres. They have a high recruitment cost and a high upkeep, so you have to balance your forces needs carefully (also recruitment limits help). Also certain garrison/standing army troops, who can be fitted intot he "free upkeep from walls" mechanism: some factions who did not habve a standing military might not have these.

Lesser levy units get recruited by the mercenary mechanism. (in M2TW this can be tailored by faction IIRC so you won't get Romans landing in Libya and hiing all the Carthaginian's vassals). They have a low recruitment cost and a high upkeep. Once again the refresh rate can be toggled.

This way even small factions have access to a large pool of cheap units quickly recruited, although it wil smash them economically to keep them standing.

Just speculatin'.