You could try taking a page out of the US civil war history books and pull a Sherman. Instead of taking and holding enemy cities, launch assaults on the enemy homelands. After taking a city, raze the barracks buildings to the ground (and anything else you feel like while you're at it) then abandon it and move on to the next. This will severely hamper the enemies ability to produce units for a rather long time and give you a chance to either push the enemy back over time or to consolidate your current holdings. Being Romani means you can't possibly retrain your main-line factional units until the marian-reforms so this is probably the best option.
Personally, I'd send an expeditionary force in through Gaul and loop around to attack the Sweboz from their northern borders. This way the Sweboz either have to pull back their forces from the frontlines and/or divert any reinforcements to deal with your forces behind their lines. So long as you move fast enough, you should be able to gut at least 3-4 cities before your forces will need to march back through Gaul and back to Italy for retraining. Just rinse and repeat and all the Sweboz will be sending your way anymore will be far smaller stacks of lower-tier units if anything.
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