You likely are using the best option for you right now. With United P.S. you are guaranteed at least Economy Plus, and you travel in a 3 class aircraft. CO also has non-stops from EWR to SNA and LAX for more options.

American and their 762 service would be a good alternative to consider. 11 non-stops a day, with 3 class service gives a great deal of flexibility (versus UAs 6 flights on P.S.) Others can give advice on upgrade availability on the two carriers.

JetBlue does have flights from JFK to both LGB and LAX. The 'even more legroom' seats have a generous 38'' of legroom, but of course no upgrades possible.

If I was travelling on such a fairly high budget, I would stick with either UA or AA. UA has the benefit of at least Economy Plus and other CO destinations from EWR. AA has the wide-body, still above average pitch in Economy, and many more flight times. You might want to compare Mileage Plus vs. Aadvantage to help make up your mind. Disclaimer: I have not travelled on either P.S. or AA 762 as I fly out of BOS.