Yep, this is a good option for reducing file size at the expense of compatibility and CPU time. Every time OP downloads a file they’ll then have to reencode the file, which can take significant time, depending on the CPU of their NAS box, the file size, etc. It’s also worth noting that reencodes are lossy, so some amount of quality will be lost (although the quality difference may be imperceptible).
If disk space is the only variable we’re optimizing for, then you’re 100% correct, but I think it’s worth calling out that this definitely isn’t without tradeoffs.
It might also be worth considering how they’re consuming this media. If the client isn’t capable of playing back h265 then this will need to be transcoded again to play it back. Many media servers (like Plex) handle this automatically, but it’s definitely worth testing this out with your setup on a couple of files before doing this on your whole media collection.
It will have a post talking about the pivot to industry, but will not have actually done the transition itself.