If I were you and want to fly on a premium cabin, try flying SWISS to LAX via ZRH on Business or fly Lufthansa on Premium Economy to SAN via FRA. N-class fares in Lufthansa are really cheap and they give %100 miles for the route flown on Premium Economy and as the short flight from BRU to FRA would be booked on Y fare class, you would earn 1000 miles for that short flight. If you have the chance I would suggest you to buy BRU-MUC-FRA-SAN-FRA-MUC-BRU in one single ticket. Just check for two stopover options if you want extra 2000 miles for the same price. This would cost around €1000 to €1200.

If you want to fly Economy, I wouldn't prefer flying TK on semi-flexible for €1000 for %50 of the miles, indeed you could fly with United via EWR or IAD and choose your fare bucket while booking on united.com. If 75% miles for all legs would satisfy, this would cost around €700-800.