Your research is unfortunately correct. 
There’s a popular unofficial integration for nest, so you can talk to the people who are using that. The integration appears to work well, but it is technically a violation of your nest terms of service, which means there is the possibility that nest would find a way to turn it off sometime in the future.
https://support.smartthings.com/hc/en-us/articles/204733384-Does-SmartThings-work-with-Nest-
As far as the TP Link Kasa Devices go, there are two community created integrations. When requires an additional server to act as a “man in the middle” and considerable technical skills. A more recent approach offers a cloud to cloud approach which doesn’t need the additional device, but you still needs strong technical skills to get it set up correctly. You can find both of these on the quick browse lists in the community – created wiki in the device type handler section under “Lighting.”
TKB Home makes a number of devices. Their Zwave devices do work directly with SmartThings, so that’s a much simpler integration. 