My understanding is that performance on LVM drops dramatically after the first snapshot due to the way it handles CoW (synchronous writes on top of your async write). Is that no longer true, or only in certain circumstances?
It seems as though the way to go would be to take a 'snapshot', back it up, and then delete it immediately; is that right?
Each time you want to a backup, create a CoW snapshot of the thin LV, then mount it somewhere and run the backup.
The "main" thin LV should be happily chugging along independently when you are doing that.
And all this is stable proven technology available about anywhere (eq. RHEL 7+).