I had this issue with my 2017 iMac Pro when I upgraded to Big Sur. I have two external monitors (Dell and LG). After upgrading, my computer would crash overnight. It seems like the computer has to be idle and/or sleeping for the problem to arise. It was almost daily crashes with a kernel panic. Eventually, the problem just... went away. Friday, I upgraded to Monterey and guess what? I'm back to daily crashes overnight. Sometimes it reboots itself, but this morning it was powered off and I had to manually turn it back on. I asked about this two years ago when it first happened and got zero response, so I don't expect anything now, but it's (still) completely unacceptable, especially after years of reports.