I have several media libraries so I set up file flows to only perform compression based on a few specific rules. I never used to compress, but 2 years ago I ran out of space and had no money to upgrade disks, so I started compressing, intending for it to be temporary until I could add space. but it became part of my servers automatic setup and it works great.
TV show episodes between 3 and 5gb that aren’t already H265 get compressed with RF21 H265, but files over 5gb get RF22. only files older than 6 weeks get compressed to give people a chance to watch them in original quality. the compression flow also includes making a stereo aac downmix audio track for added compatibility. so anything that is already H265 or low bitrate is left untouched to avoid unnecessary compression.
Movies get a similar treatment and H264 files under 5gb are ignored, 5-20gb gets RF21 and 20gb plus gets RF22. All of this is done with 10bit H265 as it tends to look a little better. the amount of compression I;m doing is pretty small, a 3gb TV show for example might end up being 2gb or so, and a 30gb movie will usually end up around 12-15gb at most. I could push harder, particularly for movies but I don’t see a need as i’ve saved 13TB so far with this setup.
If sonarr/radarr download new versions of something (a TV show gets released on bluray for example) it will go back into the loop and get compressed again, but now it will be a higher quality.
4K shows and movies are always left untouched, they are in a separate library and are only accessible to certain clients.