First, have I been waiting 9 days because AT&T denied there was an issue?...I feel like I'm starting over on this. Let me assure you the state of our line has changed little from the beginning(aside from them not fixing it and capping our speeds instead). Please re-read this whole thread above before continuing, to make sure all the context is fresh, as there's no reason we should be at a point where AT&T claims there's no issue with the line.
The DSL is losing sync. It is not merely too many devices using too slow a connection. The modem statistics during periods of instability show heavy FEC and CRC errors, errored seconds and unavailable seconds, and the line completely drops. In no reasonable world is this due to too many devices connected behind the modem. In fact, it will often come up and down during even light use. Most of those devices spend most of their time idle, and the wireless devices are mostly smart-plugs. Currently, after ~18 hours of it not keeping a sync faster than 1mb, it's back to 3mb(still capped at 3mb, never syncing higher).
Without disconnecting anything, right now I'm getting a speedtest of 2.7mbps down, 0.5mbps up: matching the current sync speed. It's sync speed(and "Max downstream available" value), had both been fluctuating between 200kbps, and 1.2mbps since yesterday evening at least, with very unstable service. It has stabilized again(briefly, I assume), at 3006kbps down, with "Max Downstream Attainable" bouncing between 5000kbps and 7600kbps.
AT&T's suggestion is a joke, so I'm not going to bother unplugging all my smart-plugs. I'll meet you halfway and disconnect ethernet though, which takes out several devices: results are 2.71 down, 0.46 up...again matching current sync(of 3mb, not 6).
....and big surprise: Line dropped again as typing this. Reconnected at a whopping 200kbps(yes, that's two-hundred kbps). I tried to run a speedtest for completeness, but it is failing to finish. After I load the page at sonic.com/speedtest and hit go, it measures the latency(37), then errors with, "could not connect". This is busy work, as a speedtest is pointless when the DSL connection itself is dropping.
After a few minute delay, and hitting the modem's button Diagnostics->resets->"reset connection", it is back up to 3mbps...surely temporary.