It's running a weird unsupported kernel, though iirc mainline 6.3 mostly works now (just don't expect hardware video encode or decode to work anytime soon, if ever on a rockchip soc running Linux)

Why not? Video decode acceleration works fine in mainline Linux on other Rockchip SoCs. This one will likely not be very different.

And there's already AV1 decoder driver posted for inclusion mainline (so the work is happening):

https://lore.kernel.org/lkml/ab2b13e31e1af52ae52b0678b0abd5d...

Not mainline yet.