I would say it's likely related to ColorOS or Play Services (or both) tracking something behind the scenes and feeding it to your ad profile. You've done a lot to try to reduce your fingerprint but it sounds like it could be something harder to track down.
Have you considered switching to another version of Android that uses microG to reduce Play Service permissions or another phone with GrapheneOS? That may be the next option unfortunately.
There are ways to successfully circumvent Google's tracking methods. It's all based on how much you care about being tracked and how much convenience you're willing to give up.