Keep using the NAS as long as it keeps being useful. Just for moving some services off onto something else, it sounds like you could just get a low power minipc of some sort and run your services there. An N100 or AMD APU would work just fine for Plex (transcoding on Intel may be a bit better).
If you’d rather go the extra mile and build a larger machine with a disk array, you don’t need anything super wild for the CPU or memory based on the services you’ve listed.
If it didn’t give you problems, go for it. I’ve run it for years and never had issues either.