I think generally keeping Heilbronn up to date but prioritizing RPM would be the best.
Spreading out our resources and eventually continue a road connection between the maps.
I suggest expanding RPM in general and focusing on the connection road. Giving the connection road plenty of companies or interesting locations to visit. Once arrived at Sinsheim (Heilbronn) we could start reworking from scratch. Deleting all old existing content from Sinsheim and rebuilding it from scratch. We'll keep expanding this while still prioritizing RPM.
In this time we could have spread out our resources to also expand the RPM map North, West and South. This way developers have enough space to work.
Calculating the development of the connection road would be around 85 km. Which suggest at least a year of development. But because we need to prioritize RPM, make it 2 years at least. It also depends on our manpower. The more developers, the more expansions we can push out in time.