This was happening to me quite a bit for a period of a couple of months. I think this was during the fall of 2020. After much frustration, the problem went away by itself (I don't know if the units did a firmware update during that time or not).
However, a little more than a month ago, I discovered that I had been overextending my Netgear Nighthawk Wifi (I have 4 teenagers, 6 or 7 minis, and 70+ devices with all of our things attached). From my questions on the Nighthawk groups, I realized that I had too many satellites for what they system was designed for. (Other things that were occuring was that Chromecast was constanting dropping during streaming).
I did some research on Wifi systems, and was intrigued with the Eero system that allowed more satellites and support for more devices. They had some deal that they would trade my nighthawk system (They gave me something for it, like $150).
I do have to say that since I went with this new Wifi Router - all the problems with the google minis and the chromecast have gone away.
I don't know anything about your wifi system, so I can't comment on that. But, what I might suggest you do is to bring one of your minis to a friends' house or your office or some other place - and install it on another network for a brief time. Then you can determine if it is a network issue or a google mini issue.
I realize that this is not a comment that you want, and it is a hassle. I had no idea that my network wifi was the issue - but I am happy that everything seems to have been resolved.
