Fyi you will not be able to do live video transcoding with a raspberry pi. I overclocked my pi4’s CPU and GPU and it just can’t handle anything but direct play and maybe audio stream transcoding, though I’ve never had luck with any transcoding period. I either download a format I know can direct play or recently started using tdarr (server on pi, node running on my desktop when I need it) to transcode into a direct play format before it hits my Jellyfin library. Even just using my AMD Ryzen 5 (no GPU) it transcodes like 100x faster than a tdarr node given 2 of the rpi cpu cores. You could probably live transcode with a decent CPU (newer Intel CPUs are apparently very good at it) if you run Jellyfin on the NAS but then you’re at odds with your low power consumption goals. Otherwise rpi Jellyfin is great.
Good luck, I’d like to build a NAS myself at some point to replace or supplement my Synology.