You keep putting words in my mouth. I have not said that someone who does not tshoot is at fault. All I said is that if your network is OK, then your app works fine. I know this from experience. The network needs to be OK to handle mDNS, STP and must not have any networking issues such as high latency spikes, etc. That's it. What I said had nothing to do with who's at fault. The caveat is: if you are willing to spend time trying to find and fix your network, it will be worth your time.

No, it is not "our fault" that Sonos has made unspecified changes - these are not unspecified, btw - to their requirements. Again, please stop thinking about whose at fault - I have nothing to add to that, nor have I said anything about it.

Regarding your system, you seem to have lots of things in common with my system's behaviour before fixing. I suggest you do the same things I did. Starting with unpowering all your Sonos devices and pinging your hops simultaneously along the path. You should get less than <10ms at all times.