AVIF supports regular AV1 videos, there is no intra-only constraint. You can try this yourself with any AV1 video:

ffmpeg -i input.webm -c:v copy output.avif

You are right about Firefox and Chrome not using hardware decoding though, as such an AVIF video plays back poorly. And Chrome refuses to load an AVIF larger than 256 MB on my machine.

AVIF video does support an alpha channel, which AV1, annoyingly, doesn't.