Nice write up!
Just wanted to be sure that you’re aware that the Groovy cloud and the IDE will be going away, probably in a few months. That’s likely why you didn’t get many responses here: most of the people who work with custom DTHs aren’t spending time on the Groovy versions anymore. 
The new platform will instead use “Edge Drivers,” but those are still in Beta, making it difficult to know what will require a custom version. But again, it won’t be a Groovy version. So it’s all pretty unpredictable right now.
The official announcement:
Announcement | SmartThings Edge for Devices and Automations
Currently available Edge Drivers (still in Beta):