Or you could wait for meteor lake CPUs which will have a HW AV1 encoder too.
Plex haven't even enabled transcode to H265 so it might not happen, but Jellyfin currently supports H265 encode and is previewing AV1 streaming. Perhaps that will spur Plex on to open up the FFMPEG encoder options a bit more and let us tune it to our needs and capabilities, but given the shift away from the piracy focused home media server they might not be prioritising it much.
In most cases yes, but hdd space is cheap enough that lossless compression is just the best option. Can always use them as originals to spin off mp3s or other compressed files when needed.
300cds would only be around 120 gigs flac compressed