The Maui flight exSMF used to be fairly straightforward to upgrade as an MVPG, which is probably part of the reason it no longer exists. I cleared it on a BA-issued award ticket a couple times at the gate, several times on revenue tickets at the gate or in advance, and could always clear it with a GGU especially if I was flexible +/- one day.

Status has been long gone for me, today I paid $750 one way (outbound via SAN) and used miles (return via SEA) for a round trip to KOA at the peak of the Xmas holiday break travel. On the return trip, I took the last two available F seats in the seat map, 9 months in advance... However, AS.com says it's still willing to sell 3 seats for a J fare, so who knows.

SEA-FLL has to be among the toughest upgrades, I have flown that flight in a middle seat exit row at the holidays several times even booking far in advance, and have never cleared a CPU on it. I cleared a GGU on it once on the redeye.

Though pre-CV19 anything east of the Mississippi out of SEA was tough with a few exceptions. I always used GGUs on those flights when I had them and could clear them.