FRA just reopened T2. DL has started flying from it as of today. FRA-DBV is served by LH and OU (=Croatia Airlines), both of which will continue to operate out of T1.

In pre-covid times, a people mover called SkyLine allowed you to do T2-T1 transfer without entering Schengen. Of course, if you're not allowed to enter Schengen/Germany, being on separate tickets would complicates things.

However, I think the probability that Germany will be open to fully vaccinated Americans by the end of July is fairly close to 1. The decision to re-open to fully vaccinated people from low-risk countries (which the US is at this stage by most measures) has been made by the EU. I strongly believe Germany will implement this during the first half of June.