I wanted to ask this same question. I think I am more like you, or should I say was like you. Is this your first time playing? It is for me. If so, you, like I was, might be overthinking things too much. Take that as a grain of salt, I just started playing myself so take the advice for what its worth.

An example is my recent experiences with Germania. I'll give you some advice that [B]is[B] worth something. Germania is hilly. Getting caught marching through Germania with siege weapons and trying to maneuver them while spear warband are charging you...

Oh, it sounded smart to line up some ballista and just pummel the warbands... ...rain and hills...

I also tried various combination forces against Gaul, like you multiple companies(units) of horse(cavalry), range weapons(velites/archers[when I could make them])...yada yada yada...

Then I decided to K.I.S.S. (Keep It Simple Stupid) and now with a standard army/force of 609 I've just beaten one force of 1900+ and then another one of 1K (mainly Gallic warband).

Here it is:

1 General & Bodyguard = 21

5 companies of Hastati = 400

2 horse (Equites) =108

1 company of velites = 80

This army or part of it has so far: took the rebel village Segesta, taken the Gallic territories of Patavium and Mediolanium, conquered Massilia and Norbo (whatever) and fought off the routine attacks by Gallic Captains including the @3K previously mentioned. They'll soon turn they're attention to Northern Gaul.

So far they have 3 experience and 2 weapon upgrades. By the time I'm able to recruit Urban Cohorts these guys will [B]be[B] Urban Cohorts.

However, all that glitters is not gold. I've recently found that I'm facing a bit of a problem using this tactic.

Money.

Too much of it. I'm trying to keep a corruption free family and having such a small military with 6 of 7 territories being Port cities...

Well, that's my 2 cents...like another post said your own range weapons can kill you, when you're fielding large forces maintaining order is... its not fun to be watching the destruction of an enemy only to realize that your own weapons are inflicting most of your casualties.