I personally dislike Plex it requires way too much horsepower and electricity to run 24/7, plus the privacy issues mean I avoid it. Jellyfin is a little less resource hungry and it's opensource / free.
I'm not, I can't stand Plex, awful resource hogging application I rid myself of it years ago, I natively play everything.
doesn't play iso's though ;)
Why are you transcoding in the first place? Try direct play and direct stream.
I run a Plex server on my PC for all the media stored on my NAS and it works terribly smoothly and is much less cumbersome and more aesthetically pleasing than Kodi.