How suitable is it for a plex server?
Do you need streaming capabilities outside of your home or only on LAN? The devices you'll be streaming to, are they capable of playing the content locally? (most smart TVs these days can play all kinds of content locally).
If your answer is No;Yes, then you can just use Pi as a NAS server and use Kodi to play content locally. That's what I do.
However, if you're already used to Plex, then the switch might be difficult, in which case and 8th gen NUC will probably serve you better.
Also, if you use multiple devices to stream, then keeping your library in sync with Kodi requires a bit of effort initially as you will have to setup a MySQL database.
I was experiencing quite a bit of latency even on LAN
That's surprising, I was using my Pi as NAS server for a while over 100 Mbps ethernet and never experienced any issues even with 4k content. I was using Kodi to play, but the content being streamed should still be similar.