Yep agreed.
You can add a retro fitting lock with a August Pro, Nuki Combo or Danalock v3 on top of your exiting deadbolt lock and then use a second one as the FE599 as the latch lock. Nuki combo (WiFi) can be integrated with SmartThings using the Nuki integration for SmartThings while Danalock and August Pro have a Z-Wave DTH integration available.
Most of these have Bluetooth versions but if you want to synchronize I would advise using the Z-Wave or WiFi version which have an integration with SmartThings.