Most likely something on that channel is spamming multicast. That kills most consumer wifi routers (in default settings). Usually something like a sonos or Google home broadcast group.

I might (i.e. I definitely do) have a non-ideal setup at home that contributes to this, with the router/WPA, a RPi running HA with a Zigbee antenna just next to it, my server in the shelf next over and a Sonos above it. Worst of all, the server is running on WiFi and it is sat in immediate vicinity to my router. Why? Because I could not for the life of me make the ethernet transfer speeds be more than somewhere around 1-5 MiB/s, so I gave up. But considering these issues are so infrequent from this location I assume it is mostly due to outside interference.

And adding to that, ....

Use an app like PingTools (Android) that can graph what is on each wifi channel. Check to find the cleanest channels in your area and configure your router to use that channel.

... this scan shows that all discovered networks are occupying the same channels (98-114 it looks like). I have still not figured out what OpenWRT option to go for (the OpenWRT One, which I had originally planned, quickly became very expensive with imports and tolls that I ended up not buying, despite having decided to go for it). So I am using the stock ISP router, which I assume everyone else in my building are as well. And I have been unable to locate an option to change channels in its interface.

If you’re curious and technically-minded, I highly recommend this write up: https://www.wiisfi.com/

That is a resource I didn't know I needed! Thanks :)

I use this:

rsync -rah --progress /path/on/source/ user@ip.of.local.server:/path/on/destination

I will try the ping next time I attempt this. There doesn't seem to be a definite time when the issues start though. It tries to copy the file over, and when it is done it continues to the next. If the first one didn't succeed, it will retry and if that also fails, it will say "ERROR verification failed - discarding update" (paraphrased) and continue to retry the next file if that also failed.

I do see some fluctuations in the transfer speeds during transfer, which could indicate the times the connection is struggling.

A bit off topic but i used to had a fiber provider which on rainy days internet would intermittently go down. I think the outside terminal was exposed or something. Also between 2AM and 6AM internet would start dropping packets like crazy.

Ah, that would make sense. I think my immediate neighbors are home, but about half of my building should be traveling for Easter (judging by the empty parking garage at least).

Makes sense that it would be able to, especially considering the flag that allows you to force an older version as the sender. Still find it strange that on equal version numbers they default to different protocol numbers. I would have at least thought there was an easy way to tell it which protocol version to use by default, but I have at least not been able to find out how.