Oh wow. That Shield Wizards looks really nice! Thanks for sharing it.
Regarding battery life: There is a possibility in ZMK to add an interrupt line to allow the MCU to go into deep sleep. It of course needs to be supported by the circuit. I took a look at their QMK firmware for the Prime61 and it seems like it also does not use an interrupt.
Yes sure. One step at a time. Mastering vim, figuring out your wm and tmux are very useful skills as well.