I am traveling on UA 51 (EWR-MAD) on 9/25/25. Booked two (2) Premium Plus (R) class. Buy-up offer to F is currently $2406, or I can use 20k miles + $450 for waitlist per person. Have UA Premier Silver status. For this route, are upgrades hard to come by from the waitlist (if coming from R class), or would I be better holding out to see if a cheaper cash buy-up offer comes around? I appreciate your thoughts.