The only thing approaching 'secure' borders in Italy would be to expand north towards the alps. The amount of territory that would need to be dealt with is quite large.
But you don't need to conquor to get secure borders. Rhome could just make Milan a puppet with a cousin on the throne or a economic satellite. Rhome does soft power pretty well and with both Genoa and Venice they could, through ownership of sea routes, make Milan subservient due to economic pressure. Maybe this war could result in some changes in Milanian politics to the betterment of Rhome so that Rhome's growth doesn't scare anyone who only cares about borders on a map.
Alternatively Rhome could just pull a Rome and have its borders be the traditional southern edge of Cisalpine Gaul. With Genoa, Pisa, and Florence in their control the mountainous border between the Po Valley and Liguria to Tuscany could suffice for a somewhat defensible boundary. It's not perfect, there is a great deal of passage opportunity on those lower mountains and the border is quite long, but at least it is a natural barrier to give a force multiplier and is easily accessible by sea from the Heartland. It doesn't really stop Milan from recovering and coming back though if they get their shit together but it's the best compromise between territorial expansion and secure borders that don't piss off the west as I can reasonably think of.
Or they could just act unreasonably and the consequences could make for an entertaining story. That's fun too.