For anyone else that has a similar issue, I think I found it. It was brought forward by my install of the "active streams" plugin. I noticed as soon as I opened it up a TON of activity despite the client being supposedly asleep, the client being one of my windows VMs. Turns out, Firefox was writing session data constantly back home, and File History was apparently writing it to my backup share, basically clogging my SMB pipe with a ton of garbage. Now I'm not sure how File History could be so busy since it's only set to run about every 15 minutes (still an aggressive setting), but I do have File History set to back up my entire user folder just in case I lose a wayward game data save.
Two solutions could be employed here. One is to decrease the frequency by which Firefox writes session data. Not a bad solution, but not a perfect fit. The second is to prevent File History from writing Firefox entirely. I employed both, since I just don't like Firefox pounding any disk, including my OS. I've yet to have another problem yet in about half a day of testing. Given that my samba config is vanilla, as was my hardware, I believe this ultimately was the problem. Someone out there likely also backs up their appdata folder or even their whole user folder, and might run into the same annoyance. I hope you find my words here.
Shoutout to JorgeB for noting the corruption, and motivating me to upgrade my rig. I cannot believe I never noticed those messages in the syslog, and of the two problems chatted about in this thread, corruption is 100% worse for this usecase than speed.
