This issue has been submitted and closed with no actual resolution previously here.
https://www.googlenestcommunity.com/t5/Chromecast/NOT-REALLY-SOLVED-Chromecast-crashing-WiFi/m-p/117...
It is also described in the following 2 reddit posts.
https://www.reddit.com/r/googlehome/comments/u2x14q/anyone_with_more_than_15_or_20_nestgoogle/
https://www.reddit.com/r/googlehome/comments/torlkp/google_speakers_network_flood_with_multiple_wifi...
There is also a similar issue that was supposedly fixed in 2018.
https://www.cnet.com/home/smart-home/how-to-keep-google-home-chromecast-from-killing-your-wi-fi/
I have a google home mini and 2 chromecasts on my network and nothing else. I discovered a few weeks ago that my mini had become unplugged at some point and I didn't notice since I haven't used it much in the last 6 months. Since it has been powered on again I've had my router, which is a Netgear WNDR4300 that has the most recent firmware, suddenly just stop working even though the lights are on and then after a short time it will restart. The router logs reset each time it reboots so it wasn't much help until I was able to look at it between the time it stopped working for internet connectivity and when it rebooted and saw that right before it stopped working there was dhcp communication with the IP for my home mini. This would seem to fit the pattern of the above issues where a device wakes up and sends out a flood of packets and in doing so crashes the router.
This issue did not occur the entire time I believe that my home mini was unplugged and I have now unplugged it again and the issue has not occurred again. Why is this still an issue when something similar has been occurring since 2018? Just to preemptively avoid the things that will typically be mentioned in order to try and blame anything but the home mini my router firmware is up to date, nothing else has changed on my network around the time this issue started occurring, and finally my network is secured. This issue occurred a few times earlier this year in around March or April when my home mini was still plugged in but then didn't occur again until recently when I discovered it was unplugged and plugged it back in. It would be one hell of a coincidence that it occurred starting around the same time described in the links above and then stopped during a time my home mini was not on and then started again when I plugged it back in.
Honestly this issue doesn't affect me much because I don't really use my home mini for anything because I now use Hulu instead of youtube tv and the voice controls don't work well for Hulu in my experience. I just wanted to post this in hopes that maybe someone will finally acknowledge this is an actual issue and commit to having it fixed so that people who this does affect can use their devices without having their router go out randomly again.
