All the data from the snap is accessible through /var/snap/lxd (mostly in /var/snap/lxd/common/lxd). If necessary, this can be manually backed up and restored as was /var/lib/lxd with the deb.
The snap content itself is in /snap/lxd/current/ and we have a number of snap set options to tweak the behavior of the snap (see LXD snap: Call for testing).
We also have a mechanism in place to sideload a manually built/debug lxd binary onto a snap install, so that we can distribute emergency fixes or capture extra debug information.