Minor update: I have been able to get CTMs, sky, etc working with Optifine in 1.12.2 - This is *super* good news and solves about 90% of my problems!
That being said, I have an old base edit that I need to find out how to implement in a better way. The files for my RezIn mod are simply modified base block class files. They change the rendering properties so that they behave like glass rather than ore blocks, meaning they handle transparencies rather than just rendering as opaque.
I have absolutely no idea how to write a mod that modified base/vanilla block files. Has anyone done this before? Looking for good advice on this.