Capture Sardinia, Carthage and Sparta before the Scipii and Brutii. Only make small inroads towards Gaul until you have a foothold in Africa and Greece. You'll be stretched at first, but you'll have extremely rich provinces that you're denying your Roman counterparts from having. They also make excellent staging grounds for future conquest and for the civil war that will come. If you can then take another settlement like Athens, you're really well prepared for the war ahead.
As for garrisons, they will drain your coffers, but unfortunately on VH you do need large ones whereas the AI doesn't need to bother. I recommend setting up permanent forts between every settlement, manned by some Town watch or depleted mercenary unit. They should be spaced so that an infantry unit can get from the settlement to fort in one turn. The forts provide a safer way for small numbers of units to move around dangerous areas, meaning you don't require every settlement to have lots of cavalry, archers and heavy infantry. The AI is less likely to attack a fort than 3 units out on the open road for example. You can then fill up the town garrisons with cheap units and have just one or two useful units per settlement, so that when you suddenly need to defend a settlement or form an army, you pull the decent units from the towns and move them between forts until they're gathered together in one place, ready to meet the enemy.
Not much else advice I can think of really, you just have to go with the flow and take any opportunities to seize profitable settlements when you can. And sometimes it's worth taking advantage of your AI Roman's plight - if you see them near an enemy settlement and you have some troops to spare, assault the city before they do. They ought to act quicker, so I have no sympathy with them for losing out to myself.
Good luck.
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