Bulgaria is actually on the Green list, not Red, but that is likely to change, not least because it seems that in a few days the UK is going to change her approach to travel restrictions. More details in the Coronavirus forum for UK arrivals. As I understand it, SOF can still be done with an easy gate to gate swap, but not with checked luggage, since the luggage cut-off time at SOF will possibly be while you are still in the air. But you do have myriad problems. These include the UK changing its restrictions (beyond the ones going through this week), Bulgaria having local problems, OLCI not working in SOF, VeriFly issues, late cancellations, PCR testing requirements, the list is endless. So while I suspect it is do-able, the risk factors are quite substantial. You can perhaps find ways of reducing the risks, for example plugging in a full overnight in SOF, since there will always be ways to get from SOF to Germany.

Have another look at this in a few days, but it's nothing like as easy as it used to be.