Big Sur, Time Machine, SMB, umount, and Safari networking issue

So this is a fun one... I have Time Machine backing up to a Synology NAS over SMB. Under Catalina this worked great from two laptops (16" mbp and a 12" mb). After upgrading to Big Sur, it works fine on the 12" MacBook. However, on the 16" MacBook Pro, whenever an hourly backup is done, Time Machine tries to unmount the SMB drive using umount, and it seems to hang. The umount process uses 100% of the cpu for ~5 minutes before timing out/dying. During this time it is unkillable (even using sudo kill -9).

For some reason, while it is in it's hung state, Safari loses all or most of it's networking. Chrome works fine. Firefox works fine. Mail works. SSH works. But Safari can't open any sites or reload pages.

Once the umount process is done/dies, Safari works normally again. At least until the next hourly backup is complete and we start this all over.

I have re-installed Big Sur. I have reset PRAM. I have reset SMC. I've tested over wired ethernet and over wifi. Nothing helps.

I've had to disable Time Machine backups for now, as I use Safari as my primary browser and having it die out of the blue doesn't work.