Here are some suggestions/observations from my current campaign. You might find them useful.

1. Decide early on if you want to get reformed units ASAP, or if you can wait until they become unconditional. You can save a lot of money by not building the highest level barracks everywhere you go.

2. Use the recruitment viewer to decide where you want to build regional barracks (this goes for any faction, of course).

3. Don't waste time on those settlements half-way into the Sahara. They take time and money to conquer and garrison, with very limited financial return. You may even wish to not bother with those settlements beyond the Pillars of Hercules.

4. If you can take Rhegion early, the Roman will concentrate on taking that city before Sicily. If you can hold them off with pikemen, you can develop your economy in Sicily without having to build walls and barracks everywhere.

5. Massalia is another city conquest to prioritise, as it will further fence the Romans in, is easy to defend, and allows you to recruit Gaesatae.

6. Pay tribute to the Lusotanans to keep them from attacking while you conquer the Mediterranean coast and build up sea trade.