You don't need to be in first class for a better flight experience. You can book with another airline that has better accommodations or get a seat with more room like comfort plus for just a little more.
True Premium Economy (such as what exists on Lufthansa, Air NZ, and now Delta international routes) does not exist for US domestic travel.
Boston (BOS) – San Francisco (SFO)
JetBlue 833 · Airbus A320
Above average legroom (33")