As it is an award ticket, and as someone mentioned upthread, LAX-FRA (LH) or SAN-FRA (LH) or LAX-LHR (UA) with the connection to TXL. I prefer one long flight with a shorter connection than two mid range flights, especially from the west coast on a red eye. If the lounge is Important to you LAX is preferable as you will have either Polaris or Star Alliance lounge. SAN will just be the UA club.

I dislike EWR, although to be fair it has been three years since I connected through there as every flight, I mean every flight was delayed or cancelled.

I like connecting through LHR and use the Arrivals lounge just outside customs and then reenter upstairs at Departures. I enjoy the walk. The Senator lounge is very nice (I find the UA club at LHR is a zoo most of the time and very noisy)

I like MUC for connections but have found immigration/security there is an issue due to some of the places I have traveled so tend to avoid MUC if I have other options, although am connecting through there next month

I flew Eurowings last month and it was fine. They are short flights with typical German efficiency. They have J - typical intra EU empty middle sear.

Enjoy your trip!