Nice build @glyph, hope it works for you! Personally I already struggle with a 4 row layout because I wanted all kinds of #i3wm specific shortcuts en bloc in one of the layers.
One thing I have on my "let's split" (really the luxury version from mountain keyboards by now but same layout) is the Ctrl/Esc double on the bottom left corner. Tap = Esc, Hold = LCtrl. It works like a charm. I really miss that when typing on other keyboards.
I also mapped the arrow keys onto HJKL positions; Or rather to HNEI for us colmak folks :P