Constant building production is all you need. This does not even use up 1/10 of your cash later on.

Constant military production is a sign of inexperience. You need very little military production. 90+% of my units built are peasants thus garrison.

Money is best used to be gifted to AI factions through hefty regular tributes.

This nicely beefs up their factions and is automatic, thus less microing. Makes you fight more interesting opponents later on. Quite nice.

You can of course bribe a lot.