Per the wiki on this thread: "GS can confirm any waitlisted instrument-supported upgrade (miles/co-pay or PlusPoints) within T-72 hours to flight departure provided at least 3 (three) J class premium seats remain for sale after the upgrade is processed and they are at the top of the waitlist. Within T-24 hours only 2 J class seats need remain. Since fall 2022, agents have been saying its valid for "one exception per PNR," thus only one-way. Having separate one-way PNRs for a round-trip may help, but some agents look for this."

And you mentioned domestic CPU's - you cannot push through domestic CPU's. Only instrument-supported upgrades can be 'force confirmed' ahead of gate clearing.

Ultimately you should expect this to continue to decline until the economy takes a turn. If United can sell out most seats, then they have no incentive, nor are company incentives set up to have them be more generous with upgrades.