Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

oh hey I've actually done this recently so a lot of this is fresh for me. Now I had a slightly different use case than you, I had a bunch of AV1 files that wouldn't run on my pi so I had to convert them to something less GPU intensive. I was finding x265 was indistinguishable for me from AV1, and had a HUGE file size drop, close to a half or even a third. x264 had a larger file size than 265 and looked worse so I don't recommend it. I did not try VP9 once I was satisfied with 265 but you could try it out and see how it compares. My recommendation is to pick one (shorter) file and run a couple of different transforms on it till you're satisfied before trying to transform your entire library

Preserving audio and metadata is trivial, just use -c:a copy for audio and -c:d copy for metadata

EDIT: I feel dishonest not mentioning the important caveats regarding my own experiments. My files were 1080p, so the difference between codecs might be less noticeable at that resolution. It was also anime, which is similarly going to be easier to compress and be less distinct between codecs. This is why I cannot recommend x264 because if you can ruin 1080p anime it'll ruin whatever you're working with. This is why I recommend picking a sample video and spend a day running a couple test transformations on it to see what you like