Not sure I have ‘the’ answer for you, but I do have a couple of comments which might be helpful.
First, your comments about lag, music dropouts, and stereo pair issues ( let’s call that 1a, 2a, and 3a) leads me to consider some difficulty with your speakers staying connected at all times to your router, through the BRIDGE. And, based on what you say, the Sonos folks also suspected some sort of wifi interference. The steps you took certainly point in that direction, but there are two missing points that I would want to address.
First, replace that BRIDGE you speak of with the newer, and better BOOST. It’s faster, and perhaps more importantly for you, has better rejection of some forms of interference. The BRIDGE at this point is known to have issues with the power supply, which can cause difficult to trace issues. Better to add a BOOST, and then retire the BRIDGE, even the ‘new’ one. Or, just drop the BRIDGE all together, and wire an ethernet cable directly to a speaker.
Second, and not explicitly stated in your second set of numbered items (let’s call them 1b, 2b, and 3b for additional clarity in referencing), I don’t see anything about refreshing your SonosNet network. Perhaps you weren’t asked, because doing so for 15 devices would be slightly time consuming, although simple. It’s also a much lower chance of being the issue, but still... Unplug all Sonos devices from power. While they are all unplugged, reboot your router. Once the router comes back up, plug back in the Sonos devices, starting with the one you have wired to your router.
This process is more temporary, you could easily achieve the same on a more permanent basis by assigning all Sonos devices reserved IP addresses in your router’s DHCP table, instructions for your particular router would be in it’s manual.
Those would address potential issues in your Sonos network, but there is always the possibility that the issue isn’t in your network, but at Spotify, and all the messing around you do won’t ever make a difference.
It is important to note a couple of things. First, most streaming companies have a specific feed for Sonos, not the same one they use for their own apps or the web. Mostly this is to make tracking where the people are coming in from, but people sometimes say it works in one place, but not my Sonos, without understanding it isn’t a single unified source for the data. Also, different streaming companies have differing tolerances for disconnections, so something that causes Spotify to drop may not cause Pandora to drop, which makes it hard to rule out an issue when one service is working, but another drops.
Hopefully, some of this knowledge is useful. The Sonos folks, with their access to diagnostics, would normally have greater insight as to why you’re having this issue. I’d be tempted, the next time it happens, to submit another one, and call back in.
