Happy to help here. This is where we get into the complex world of points.

So with United, you're going to have a very hard time finding a first class flight for Zürich to Atlanta route in September. The only possibility is Lufthansa first class with a connection, but Lufthansa doesn't release their first class product to partner Star Alliance airlines (like United) until a few days before departure.

United does not have international first class anymore. SWISS almost never releases their First Class to partners.

You cannot book Air France first class with United because Air France is part of SkyTeam and United is part of Star Alliance.

You cannot book British Airways first class because BA is part of OneWorld. Not that you would want to, British Airways first class usually carries fees around $800-$1,000.

If you are married to just United, then the best tool is the United website award search. Roame shines when you need to search across multiple airline website at the same time.