I was thinking for the DR-like feature, it could be:
• Initial VM backup & restore as normal
• Volumes of DR VM in DR state (using Longhorn's DR feature). This way the volumes stay as up-to-date as possible. Shouldn't be an issue as the DR VM would stay off anyway.
• When the DR VM is activated, the volumes are moved out of DR state and are used.
When the DR has served its purpose and it's time to return to prod:
• DR VM is backed up
• Prod restores the backup into the existing (but outdated) VM
• Prod VM can restart running
• DR VM (which is not in DR mode) gets destroyed and step 1 is run again. I say this because Longhorn doesn't let you turn an activated DR vol back into DR