Unless you want to do lots of connecting flights a return nonstop Transcon flight in AA Flagship First will get you 420 TP. Available around £1450/£1500 for your dates. SFO or LAX (SFO often has better availability.). Plus on the Transcon flights you get fully lie flat seats instead of the usual US premium economy style seats sold as First.
Transcon Departures from BOS usually slightly cheaper and a separately ticketed domestic First JFK-BOS positioning flight would get you the additional 80TP to take you to 500TP.
If you book on a single ticket a connecting BOS-JFK-SFO Flagship First you would earn BOS-JFK 40TP domestic first plus JFK-SFO 210TP Flagship First so 500 TP round trip for circa £1450. Add a cheap positioning flight to BOS or catch the Acela up to Boston.
Probably cheaper options available and TP per £ spent isn't great but for comfort and simplicity a Transcon Flagship First is an option to consider.
Looking for this route but the cheapest I can find BOS-JFK-SFO one way is £1360 or return £1910. Which dates did you try with?