More updates:
Considering @RHBH has got his PPPoE connection working just fine on a DIR-882 on r13938-15f585afc5, I flashed the same image to test if it'd work.
Spoiler: it didn't.
But the "great" news is that, for the first time, one of the lan interfaces showed intermittency on this version! So I believe there's definitely something wrong with the mt76 drivers - RHBH is having wifi issues.
Granted, I flashed a DIR-882 firmwate on a DIR-878 device, but it's very nearly the same hardware and I'm writing this though it, if it were incompatible it'd probably have bricked the device or at least LAN wouldn't work at all.
I'm trying a different setup to see if my connection becomes stable: I configured the ISP modem as router, and OpenWRT DIR-878 running DIR-882 r13938 WAN through DHCP, effectively doing double-NAT.
Here's the system log that shows the newfound LAN intermittency: https://pastebin.com/CqFNPmEu
Further testing connecting the desktop to the modem directly and dialing PPPoE via Linux shows everything works fine, except when OpenWRT is involved.
Should I file a bug report at this point?