Simple Nightly Sync to USB - How?

Snapshots don't actually take any space unless you've had a lot of churn, so I',m unsure what your issue is with the snapshots.  They occur on some backup jobs to insure nothing is changed while it's being backed up (it backs up from the snapshot).

But if you instist on doing it, you can set up the source as remote and use the IP address 127.1.1.1 (called LocalHost).  This will make the backup a "pull" from what it thinks is an external source, where is cannot perform a snapshot.  Using rsync is the best for a sync, as it has an option to delete files on the destination that were deleted on the source.