Delta has blocked all middle seats from assignments. You can not choose a middle seat in their seating charts. They are also only selling First Class at 50% of capacity and economy at 60%. On planes without a middle seat, they will block random window and aisle seats so there is separation on the plane. The difference between Basic Economy and Main Economy is basic economy has no advance seat assignment, no baggage allowance, and they board last, so overhead bin space might be limited, but in the current situation, I doubt that is a problem. Basic Economy also not does not qualify for upgrades for medallion members.