First I am not tech-savvy, so please assume I know nothing.
I was gifted a strong PC:

I don’t game is why graphics aren’t good.
Currently my media is stored on a QNAP NAS TS-451 that I got 5 years ago. Plex doesn’t run off that, it’s just like a nice docking station for 4 8tb hard drives (I barely know how to use a NAS).
I also just moved to a place that has fiber, gigabet internet, 1000 up and down.
I want my Plex remote shares and my own usage the best, little buffering, lots of power, etc. etc.
My tech guy said that since I have fiber internet I can just keep using my NAS and it’ll be the best? Despite not knowing how that works, I don’t understand why only a faster internet means my old NAS is all I need. I thought if I have the media stored in my new PC it would be a lot better than a 5 year old NAS?
Advice: given what I’m looking for, are there benefits to moving the drives into my own PC? Or is what he said about sticking to my NAS correct? If you have any other ideas that could work here, please let me know. tyvm!