11/8/20 should see the first ex-BER BA flights. A few of us are booked on BA993 departing 12:20.
With regards to the lounge it could end up being rather comfortable with BA departures in the south corner of the C gates (apparently C17-C19) on the non-Schengen level. The lounge is right there, although officially only listed as reachable from the Schengen level. I've seen claims that it will allow for direct access to the non-Schengen are, though.
I'll be on the first BA flight and will report back. I wouldn't count on all lounges being open at that point.