AV1 is supported upstream on RK3588 but not RK3576; the latter has a different (all new) IP block and nobody wrote drivers yet.
And the train is indeed a little quiet. Curating a working image still requires a large number of patches and as some of the in-flight series have iterated/progressed the number of conflicts between things has gone up, and resolving and figuring out how to solve those problems requires a level of effort that I don't have time and motivation for at the moment. Things are at the stage where the easy option is just leaving things as-is until more of the major bits are merged upstream and the overall patch count reduces to a more manageable level. That probably doesn't play well with the likely timeline for K22/LE13 though. Catch22.
LE runs happily on 2GB boards unless you're planning to run flatpak things or other services in the background. 4GB is fine.