So...yeah. Looking at file size, it clearly beats older 264 or even 265. I don't mind if my server is going to have to transcode for most clients, I think the size difference in size might be worth it. But not sure which groups I could focus to look for these AV1 releases, seem they're quite scarce still?
A properly muxed mkv will display the resulting audio bitrate. And if you use opusenc, it will embed the encoder settings in the track.
I think variable bitrate is preferable. With a variable bitrate you don't have a single, specific, telling bitrate show up. In the end you depend on the encoder doing decent work. Which group names can be useful for, to identify and revisit good ones.