That’s a lot of different questions, and I’m sure you’ll get a lot of different answers. 
The official announcement of the roadmap is here:
But note that there aren’t a lot of details. Details have been coming out piece by piece, and you will find them in the developer section of the forum.
We’ve gotten a lot of information on how cloud cloud integrations will work, and a recent detailed breakout on how “direct connected“ (Wi-Fi devices that use the smartthings cloud but don’t have their own cloud) will work.
But as yet we have not been given many details on how “hub connected“ Devices, meaning zigbee and zwave, are going to work, or even if it will still be possible to do custom DTHs for these.
OTOH, if smartthings really got their act together and supported more of the third-party standard functions, we might not need custom DTHs for hub connected devices. 
The main reason for custom DTHs for Z wave devices, for example, is because the smartthings Z wave implementation has not fully implemented features like central scenes. So people are always trying to work around the limitations of the smartthings platform. Fewer limitations would mean fewer workarounds were needed.
Anyway, you’ll find the discussions about the various development options in the following section of the forum
As far as alternatives, there has been tons of discussion on that. You can try the following threads which are specific to the announcement of the hub retirements:
https://community.smartthings.com/tag/eol_hub_migration
And you can also check the following section of the forum:
Not sure any of that will really help immediately address the feeling of falling off the ledge, but it’s a place to start.