Check out this article from SplitKB, it has some ideas and sample code you can start with:
https://docs.splitkb.com/hc/en-us/articles/360010513760-How-can-I-use-a-rotary-encoder-
Personally I have mine set to switch windows mostly, but lately I've preferred using a set of custom keycodes on a nav layer for that. I have to hold down a key to activate the layer. Tapping one key sends tab, the other shift-tab, and both hold down alt if it isn't held down, starting the window switcher. Releasing the layer key releases alt, implicitly selecting the current window. It's faster for me to do this than to move my hand to the knob.
Nice build!
What do you think of the 3D printed case? I'm currently interested in 3D printed cases with steel plates, they're just like prebuilt keyboards only ergo. The case for this version has an integrated plate, so I can't drop steel into it.