AV1 is only just appearing in TV chipsets, and software support and stability will lag behind there for some time. if you only use your videos on a modern PC or a new-ish phone, then sure go for it, it’s pretty great.
Personally though, as good as AV1 is, I’d be avoiding it for something like a plex/emby/jellyfin library purely because while computers and phones now have decent support, many TVs and streaming boxes do not, the software on those that do is lacking support or is patchy, broken or unstable, and you can run into difficulty even transcoding those files for playback on unsupported devices because of the transcoder backends having their own support problems depending on your server hardware, operating system, and server software choice.
H265 10bit is the current best for those sorts of media libraries, just about any TV or streaming box from the last 5 years will support it just fine and it is still somewhat easy to encode with hardware acceleration.
I recently ran some of my less critical libraries through fileflows to convert a small subset of oversized H264 files to H265 10bit and with roughly 17000 files processed in those libraries I’ve saved about 5tb. that is skipping small H264 files and files already in H265, and has a few encoding tiers based on file size and some handling of reprocessing outputs that end up larger than the original, which can happen with the lower RF values that I am using.
Output quality has been perfectly acceptable, but i still have many thousands of files that I would prefer to keep in the highest quality possible, regardless of file size.