UA counter at SFO International terminal. Have your car drive to the far end of the dropoff area, the counter is the farthest North and closest to the TSA checkpoint (I think area 1). At that hour you should have a fully staffed 1K desk though you may still end up waiting 10-15m depending on the competence and how many show up before you.

You should be able to get the BP for LH but sometimes they can't print it. Don't worry, there are usually convenient LH desks on arrival at FRA that are far more efficient than SFO and can print your pass instantly.

If you checkin online be careful with the dates. Last time I did this online I ended up checking in for a flight that departed the day before. Not sure why the system permitted that but it caused some confusion when I hit the SEN lounge at FRA. Fortunately they were able to sort it out and get me a BP for the correct flight.