Alright, network behaving. Everyone now agrees that the edge router is the root of the spanning tree.
Wired performance now 9 ms ping, 86.13 Mbps down, 12.48 Mbps up (speedtest.net, averaged of 6 tests, 2 each to three servers).
Honestly, I don’t think diagnosing spanning tree broadcast storms and changing the default value advertised by the router over RSTP is exactly consumer level skill.
Plug and play my fucking ass.
And now, six hours later, we return to our planned diversion of playing with 802.11ac on multiple ACs with the same SSID on 2.4 GHz and 5 GHz.