Currently, there is no easy way to compare either two snapshots or to compare one snapshot with the current edit. Both would be very convenient when working with snapshots.
A possible workaround is dragging one edit step from the protocol into the "Before" pane of the Before-After view, but you never know if you picked the right edit step (making a snapshot leaves no trace in the protocol pane), and also you cannot compare two snapshots that way. See discussion in this thread: https://community.adobe.com/t5/lightroom-classic-discussions/snapshots-how-to-compare-them-side-by-s...