As WineCountryUA suggests, the ANA tool is great to find what is available. I did a quick test search and I see good availability in April, especially mid-week, on LH in business and sometimes UA in first on the 747 for the direct SFO-FRA & FRA-SFO. Also economy seats available on LH.

If the OP is a bit flexible with the dates, the desired trip should be not too difficult to arrange N/S. Willingness to connect will help if flexibility in travel dates is lacking.