AV1 has been done ONLY because Collabora had a funded project to upstream support for a Mediatek chip and getting a new codec merged is easier when you can demonstrate multiple implementations; and at the time RK3588 was the only other almost-usable chip to work with (and their main interest has been the bits needed to make it usable for that use-case). Now that’s been done the rest of support is largely down to Collabora pro-bono work and the usual “send samples out and pray the community sends patches upstream sooner than later” model. Hence you’re a year on from RK3588 shipping and there’s still the complicated bits nobody in the community is capable of (GPU) or fancies doing (codecs) missing.
The reason I’m a Pi fan has little to do with their hardware which is always a deliberate compromise and thus demonstrably inferior in numerous ways to a bunch of other chips and boards in the market. I like them because they’re the only SoC vendor who truly gets software support. Since RPi4 came along they’ve embraced upstreaming (outsourced whenever possible) to reduce technical debt and improve support for Pi hardware beyond their own distro, and I can post an issue on their GitHub kernel repo and 24-36 hours later a fix has been merged and the ticket chat has been directly with the Engineers who coded the fix. Their level of advance software readiness for RPi5 meant creating the first LE image (as functional as existing RPi3/4 images) took less than an hour including compute time to compile everything from sources. RK3588 is the superior chip but hardware is only part of the story.
NB: Some LE devs (some of the less active ones though) are taking an interest in some of the missing RK drm/media/codec bits so that will hopefully show some progress over the winter. It will be nice to have LE images for the recent(ish) RK chips available once more bits are done.