Instead of the ICE that on the FRA - Frankfurt main station route often has standing room only, I would take 1st class on a RB/RE train. The short hop is not long enough to enjoy the comfort of the ICE and the 1st section on the regional trains is almost never crowded.

Generally when travelling local distances with luggage I'll gladly pay the few extra euros for the RB/RE 1st section. There is storage room in front of the cabin.