Transcoding anything >720p is painful.
I run ancient hardware for desktop/laptop >10yrs old apple stuff running linux. I consume media mainly via rpi4 or android.
What’s a minimum level system capable of trans-coding 4k video to x265 in at the very least real time? Is there a tiny trans-coding device out there somewhere?
Would a NUC do? How old or new to churn out 4k x265
Can I avoid hardware? Are cloud gpu’s a thing?
Try handbrake maybe https://handbrake.fr Might not be exactly what you want but something to look at while you wait for other answers.
If you are actively making 4k videos through, I think it might be time for you to upgrade.