Check out this:
Plex NAS Compatibility - Google Sheets
Your old NAS was way underrated to run anything except to push as direct play.
Your new proposed NAS should be pretty good but I would check out how you package your media and how your devices are set up. Many people still have problems with a good NAS because they do not prepare their media properly or are not using the receiving devices in the optimum settings.
Also for 4k ALWAYS refer to this post:
[INFO] Plex, 4k, transcoding, and you - aka the rules of 4k - General / Tips, Tricks & How-Tos - Plex Forum
Keep in mind you will need a Plex Pass to enable server hardware transcoding.
Lastly, per the chart the DS418 is actually rated lower and less capable than the AS5304T.
