Why not get a Pixel? OP 9 Pro has a snapdragon 888, a powerful yet incredibly demanding chip which will lead to overheating and poor battery life. Pixels are known for their custom rom support (where you can flash a custom rom with root access) as well as Graphene OS, ironically the ultimate degoogled android experience.
Price range for used pixels on amazon:
80-120 USD Pixel 4a
90-135 USD Pixel 4a 5g
130-150 USD Pixel 5
130-200 USD Pixel 5a
185-260 USD Pixel 6
210-230 USD Pixel 6a
270-325 USD Pixel 6 pro
You could also get a 7/7 pro, but Best Buy just finished up with a sale for them at 350/600 which is way better than the used market.
Buying the OP 9 Pro would be a huge mistake. If you don't mind a few scratches, the $185 Pixel 6 sounds like way to go, or you can buy the T-mobile variant for cheaper here.