Great results so far!
I recommend looking into custom attributes if you haven’t already. If you add a float attribute to one of your joints in Maya and add keyframes to it, it will appear as a curve track in Unreal if you have Import Custom Attributes enabled. (should be by default) You can then use this curve to drive things like material parameters and other properties. It is really powerful. Considering you are already using a lot of tricks for your rigs, I think this can be really useful to you.
Try experimenting with wrinkle maps and perhaps blood flow (diffuse) maps for your expressions. It will make your face rig look more organic.
One last recommendation is to learn the art of “less is more”. Your deformation rigs are amazing, but I fear that the abundance of joints may cause you headaches when you get into gameplay later on. Consider making your current rig the cinematic / high quality version. Then create a Level of Detail which does not go crazy with deformation joints. You will be amazed how much you can cheat without players noticing that some deformation is missing when the character is not filling up the screen. Don’t be afraid to sacrifice some perfection for performance.