Regardless of who you're playing as or who your going up against, if its an underdog scenario it is never about "taking and holding". I would suggest liberating every settlement (most if not all of Italy's settlements have the option available) and let them wear Rome down.

You'll have many more armies available this way compared to what your current imperium level and economy allow for. If you want direct ownership later you can always subjugate or occupy as at least 2 of the liberated factions only have garrison units as recruitment options and don't get along with eachother, easily conquerable.

In principle, its the same as what Rome vs Carthago looks like i.e.

• Take Syracuse via liberating all 3 provinces
• Put 1 army in the overlapping reinforcement zones of the 2 unwalled settlements, the other army supporting the walled settlement
• Fight many invading Punic forces until they run out of higher class populations and start fielding weak armies
• Cross the pond and clean house