You won’t need custom code for the Leviton devices. If a device is on the official compatibility list, it is automatically recognized by SmartThings and it should work fine.
Custom code is only needed when manufacturer has a device that has not yet been officially certified to work with SmartThings. Like the new homeseer switches.
Again, you’re fine with the set up you described in your first post using those specific devices. They already use instant status. They’re already on the official compatibility list. They will work just fine.
If you want to talk about any other device possibilities, please list the specific brand and model, because the features do vary significantly.
You might also find the following thread of interest. The discussion of switch features starts around post 27.