Bumped up some my speeds by a bit by replacing cables.
TL;DR - If you're noticing throughput issues or bottlenecking, check the cables going to and from your router; try swapping them out for known good cables.
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Ok, first off; yesssss I get it "duh" but hear me out because I overlooked something so simple. I have a 5 gig connection running into a 10 gig port on the Ziply provided Aginet router going to a TP-Link 10 gig switch out to 10 gig ports on both my computer and my NAS.
For the longest time I was experiencing slowdown, mostly transferring and viewing files from my Nas either via VLC and Plex both internally and remote. I had run all manner of tests, checked and changed all of the cabling beyond the router. Eventually I just wrote it off as either the connection LAN being saturated and/or streaming being limited due to HDD speeds.
Well, today I decided to change out the cable running to my switch thinking that it may be too long or faulty. As I was doing so I noticed the other 10G port on the router which goes to the ONT. The cable was, well, kinda cheap looking. Grey, plastic, and flat. So I swapped it out with a new short cable I had available and everything wired is much faster and more stable. Transfers over the network to the Nas are doubled but more importantly they no longer start, then drop to 0 for a bit, then start transferring again only to drop to zero, etc.
Looks like the Ziply provided ethernet cable from the ONT to the router was the culprit. I hadn't thought to look at the ONT cabling because I guess I internalized that the installer would have used appropriate cabling (or that there wasn't anything wrong with the cable.
Posting this because I'm laughing at myself; though someone may have seen a similar issue despite how obvious the solution is.