OP is planning to try to cover too much in a six week trip anyway.

If OP wants to pursue this, pick a single country (ideally with nonstop flights to/from USA) and plan to get to know it well by visiting smaller cities and towns as well as the big tourist sites.

The obvious choices would be France or Germany. OP already has RT tickets directly into France, but it might be worth exploring the DL "quarantine free" flights to Italy with special COVID-19 requirements and procedures.