So I think I have an idea of your problem… your NAS is just plain to old/slow for what you are asking it to do.
Typically the spinning icons you sent me a picture of are caused by 1 of 3 things related to your DVR recording.
Live TV & DVR (Set Up and Manage)
https://support.plex.tv/articles/225877347-live-tv-dvr/
Since the jobs didn’t finish when you stopped/restarted, it is trying to continue to finish them.
Even on my system (Ryzen based TS-877), it does take a little while to run remove commercials/transcode the items. I also see the occasional failed thumbnail message. No biggie to be honest on that one.
As for newer/beefier NAS, the Thecus isn’t really using anything special (using same Intel CPU’s others are). They are probably a “minority” in terms of % of Plex NAS brands vs. Qnap & Synology.
I would stay with WD Red or Red Pro as those are the NAS rated drives versus Gold.
If you are looking for a newer NAS to replace your current one, since you have 4 tuners, I wouldn’t go down to a 2-drive NAS if you frequently have several programs recording at the same time. You need the IOPS the additional drives give you.
The N3150 CPU in the Thecus N2810Pro is already like 1+ generations back of where Qnap/Synology have gone.
If you are looking to do Plex and want some lifespan for Transcoding, I would look at say the QNAP x53Be family as a starting point which have support for HW-based 4k transcoding.
An easier/cheaper option would be to keep your QNAP NAS and use it strictly for Plex Storage.
Buy an Nvidia ShieldTV + USB Thumbdrive (doesn’t need to be too big) and run your Plex Server/DVR usage on that (make sure and have hardwired 1GbE connection for it). You will point the Nvidia Shield TV PMS server back to your QNAP NAS for all of it’s Library storage, etc.
The Nvidia Shield TV PMS supports HW transcoding, will connect to your HD HomeRun, etc.
You can get the Nvidia Shield TV + Thumbdrive for under $200.
See here for more information re: PMS on Nvidia Shield TV. https://support.plex.tv/articles/221099988-setting-up-and-managing-plex-media-server-on-nvidia-shield/
I think that is your best option as your QNAP I assume is running fine for your other needs.