I think using OpenGL will be even harder than DX8. Your point with the documentation is spot on: it will be hard to come by. As for TLS/SSL third party libs: I had the fun opportunity to implement/maintain the backwards compatibility for a piece of software for Win XP/Server 2003 ~3 years ago. We had to jump some hoops with a custom compilation of OpenSSL to get the parts of TLS 1.2 we needed in Windows 10. I can say with confidence, that that route is a dead end for even older Windows versions.