The /proc issue is a LXCFS 3.1.1 bug, not snap specific, you can read more in the many other forum posts about it or on the Github issue here: https://github.com/lxc/lxcfs/issues/295
The NFS issue is interesting, do you have any idea why it’s hanging?
Is it because the backup server itself isn’t responsive at the time, so any filesystem access under its mountpoint is held by the kernel or does it look like something else?
As for controlling refresh, you can configure a time during which those happen:
For production environments where you may want to do staging on a number of machines before updating the rest, the snap proxy lets you do revision overrides of any snap that uses it: https://docs.ubuntu.com/snap-store-proxy/en/