I would say that in terms of actual rendering, you just need to be familiar with OpenGL. The non-generic part of rendering are mostly changes in the particle system and rendering system, which is not that huge of a deal to adapt to when Forge updates (might takes a few days to learn, but you will get there).
Other than that, just check out the vanilla rendering examples (including usage of buffer builder, tessellator, etc) as well as the documentations on the model system. These change a lot between Forge versions.
As for LTS, modders took a vote on that and 1.14 is preferred by most people.
Modern versions of Minecraft is actually more stable and have better performance (compare the code with, say, 1.7.10). A lot of popular mods in 1.7.10 and 1.12.2 are rather... terribly written in terms of code (check their sources; most of them are open sourced). There isn’t any reason to not move on to newer versions apart from the large player base (which eventually moves on to newer versions anyways).