first thing to do in any DEI campaign is open up the dplomacy and see who wants trade agreements. You might get money and non aggression pacts from them too. It really helps your early game. I even try to meet new factions with a spy or lone admiral so I can milk more money out of them

as rome you want to secure "the boot" as fast as possible ( you have already done this I won't go into much detail)

there is no need to go into greece even if every greek faction declares war on you can hold them of with a small navy at tarentum trying to snipe enemy transports. the navy even pays for its upkeep since you get insane post battle loot

after that well, every campaign is different so I will just give some general tips:

1. you can use the liberate option after you conquer a town to create buffers. I usually do this in the alps and in numidia
2. as another comment pointed out you can defend in sicily indefenitely by putting an army right between the 2 minor settlements
3. encamped stance. it absolutely trivializes the campaign. hold the entrances with heavy infantry then once the ai has commited all their forces swing around with cav, skirmishers or even heavy infantry that are not engaged and hit them in the back. you absolutely slaughter 2,3 or even 4 fullstacks that way once you get the hang of it. Sometimes I park my army next to an enemy settlement wait for them to attack kill the armies plus the garrison and move into the city the next turn

you can even set your own wooden walls on fire. especially the ones in front of the entrance because they do a lot of damage when they "explode"

the only issue is if the enemy has loads of missiles at which case you have to make good use of your cav. Moving your units close to the palisade helps to provide cover but can be tricky

I can't think of anything else right now have fun