If anything putting your Sonos gear on one channel and your WiFi APs on the other two (1, 6, 11) open channels should reduce interference between them. The WiFi devices and Sonos should not impact each other as long as you keep them on non-interfering channels. A few 2.4 GHz devices don’t behave well and splatter across several channels and don’t share well, baby monitors have the worst rep.
My setup was not happy having all the APs on the same channel to keep the Sonos happy and it impacted other devices that were moving a lot of data, HD TVs in particular, they seem happier when split to two different channels and APs. I’m not sure if that is a current requirement or not, been a while since I messed with that.
A Boost should not cause any more problems than any other wired Sonos as they emit very similar signals. Adding a Boost or wiring another Sonos does move all your non-portable Sonos off the WiFi and onto SonosNet so that might be the source of your issues if you didn’t set SonosNet to an open channel.
Sonos over WiFi isn’t just about signal, you can have what looks to be a perfect setup but that can change minute to minute as the Sonos mesh re-configures to meet changing conditions. If you can get a consistent problem submitting a diagnostic to Sonos and asking for a review might tell you more than we as users can see of the Sonos internal status.
I doubt you had too many WiFi devices, unless one or more are not behaving properly they should happily (if a bit slower than the maximum rate) share. I usually run wired but I’ve fooled with WiFi mode from time to time and my 18 Sonos (2 in surround groups) seem to have no issues as long as the WiFi AP is set to meet their requirements. They are sharing with 23 other WiFi devices, most not moving a lot of data, with no issues. Now if I move my security cameras to WiFi mode the channel gets busy enough other devices are horribly slow and Sonos starts to get dropouts but that is to be expected as the channel utilization is above 80%.
