Imagemagick that's packaged in Debian doesn't have avif support unfortunately. They somehow split between version 6 and version 7 which is confusing, because both are progressing in parallel but they don't have feature parity.
Are you talking about "stable" repos? Do you know anyone who sticks to those, for personal use?
(A lot of people use "testing" for reasons I have not been able to comprehend. The main difference from "unstable" is it takes a minimum of 2 weeks for fixes to show up in "testing". Or did, last I checked.)
Ah, I didn't realize that! Looking at this [0] issue thread, it seems like the minimum version with AVIF support is v7.0.25. The suggestion to use Docker in that thread might be useful for you as well, if you are interested in a couple quick tests and are already set up for containers.
I can try getting version 7.