I've had several system freezes (goes from sluggish to frozen within just a minute or two and panics) related to copying large files or large numbers of smaller files to and from SMB-mounted shares. It doesn't seem to matter whether I CMD-K or add them to automount(my preference). I ran with this configuration with multiple SMB share mounts on Intel and pre-Big Sur versions without this problem for the better part of 10 years. It's definitely new. Unfortunately I don't have another Big Sur-capable system to rule out either M1 or Big Sur. I have an M1 mini on Big Sur and my watchdog panics/crashes with smbfs.kext are perfectly described on the bottom of this page: https://bombich.com/kb/ccc6/macos-big-sur-known-issues On that page see: "Apple's SMB filesystem client causes system stalls, leads to kernel panics" and mouse-over the "watchdog panic". Based on how few other search results I get about this I am worried that nobody is taking this seriously or it's a nasty edge case that isn't even on the dev radar. I hate to say it but I'm glad it's happening to/with Bombich CCC so maybe it might get some attention.

If you leave it for a while, does it start responding again? I've been having this problem a lot. I actually have a TextEdit window with a 5.1 MB file open right now. If I click on the scrollbar to move it a page, drag the scrollbar up or down, or use page up/down or arrow keys it "beachballs" for several minutes sometimes and uses 98-100% CPU ("Not Responding") in Activity Monitor. The file itself is just a 138K line log of basic email headers (From, Date, Subject) in plain text format. I'm on a late 2016 13" MBP using an OWC Thunderbolt 3 dock to an Apple Dual-Link DVI connector, not HDMI specifically.