Since you need several streams of simultaneous transcoding and tone mapping, I think that is definitely a good reason to get a separate Plex server.
If you insist on the NAS being the server your Intel options seem to be limited … Or, you will have to wait on Plex’s full support for AMD.
Alternatively, get any NAS you want plus a ~$200 Intel mini system for your Plex server. (If you have any spare PC hardware around you have even more options.)
I should point out though that a Windows server cannot do Quick Sync hardware accelerated tone mapping… A Linux server is required for that. Or, go Windows and Nvidia, but there are simultaneous stream limitations imposed by the Nvidia drivers.
