This is the main reason I too tend to gravitate to UA for most international travel. Only way round that is to (where possible) buy partner flights through United and that way you have the flexibility and ability to change. That though, generally, restricts you to preferred partners, as non-preferred partners generally overpriced on United, but sometimes can get reasonable fares on non-preferred partners but may have to go through Google flights to get them to show up. Other thing that can work sometimes is to call as may occasionally can get an agreeable agent. Couple weeks ago I bought a BR ticket on 016 with no UA segment over the phone .

This approach though would defeat your objective of maximizing PQP as you end up with spend based PQP, but given the PQP caps, it seems to me that on practical basis, it would be the exception rather than the rule that buying a partner ticket on non 016 stock will lead to substantial increase in PQP per dollar spent - those very cheap premium cabin tickets are largely theoretical (other than a pure PQP run) as they are generally impossible to find for routes and times that one wishes to travel. All the times I have flown partner airlines on non-016 stock in 2023, I ended up with well below 1 PQP per dollar spent, sometimes lower than 0.5.