Smart speakers

Hello together My four HomePod minis are constantly having connection problems. For nine months now, I haven't been able to get to the bottom of the problem. Sometimes it works, sometimes it doesn't. During stereo listening, one speaker suddenly cuts out; individual HomePods suddenly lose the connection; HomePods play music but don't show this (with the light or in the Home app); sometimes they can't control the volume of the others or don't see the other HomePods in the network; ... The problems are quite diverse. I have already done quite a bit to get to the bottom of the problem. I have already reset all of them several times (HomePods with Mac, router, iPhone network settings); taken individual HomePods out of the system to rule out that one is broken; placed all HomePods very close to the router; run only 5 GHz or only 2.4 GHz; set up fixed IP; ... Does anyone have any other ideas?