So, i've learned a number of incredibly useful techniques on this board... what else am I missing? Now that i'm comfortable with the above, what should I look at next?

Advanced masking with channels is very useful. Learn how channels really work in different color spaces - RGB, CMYK, Lab. Learn how you can mix them with apply image/calculation commands. There is a lot of advanced things can be done with channels besides creating very precise mask. You can create drastic color shifts for creative purpose. I would also recommend to really learn how blend modes work as well, because you'll be able to predict it better what result you'll get when using apply image/calculation commands since it's quite a challenge sometime.

Color theory. One of the most common questions asked in this forum is "how do I get this look?" Which is usually in regards to the images color palette. If you know some color theory and how colors work in Photoshop, you should be well prepared to get any look you want.

Other than that, practice. It's one thing to know of a particular technique, it's a whole other thing to be any good at using it. The more practiced you are, the more proficient you are.