If you’re mainly using this server for plex, then a 13 series Intel with quicksync will be the way to go, from what I read the newest generation of quicksync is really good for transcodes. That and more RAM, can never have enough ram!
Is plex transcoding what’s making your server struggle? Even with older gens, I would think using hardware transcodes would keep the cpu load light. At least that’s what I noticed when I dropped an nvidia card into my server (no integrated graphics in my cpu)