Hardware under/overkill?

[TheDreadPirate]

That should serve your needs as long as you don't need tone mapping (converting HDR to SDR). The 12100 should be able to handle a single 4K HDR to 4K/1080p SDR transcode.
For plain SDR to SDR transcodes, my testing reached 12 simultaneous 1080p H264 transcodes. If you need a bit more transcoding power, Intel Arc GPUs are fantastic. They're cheap, readily available, are plug and play with Linux + Jellyfin, and are transcoding champs.