Here's an idea, though I don't know if it would be practical.
I assume that at some point everything that goes into RHEL and its "official" updates travels through the Stream ecosystem beforehand.
So what about the idea of maintaining (somewhere) a list of official updates to RHEL as they are released, and then have some kind of a dnf enhancement or script that reads that list and updates a local Centos installation using only the rpms on the list and ignoring everything else. Stuff that's released to Stream would be downloaded and installed at the point where it has been released for RHEL and not before.
That would (I think) keep your Centos installation in sync with RHEL.