This might not be the answer you want…
Take a look at the pinout of the Z-axis driver IC. You can tack on a wire on the Dir, step pins, GND is board ground, lots of places to pick that up. It shouldn’t be too hard to hook up a geckodrive (https://www.geckodrive.com/g201x.html) , and I think I’ve seen a couple people that have done so. All that said, if you’re going to do one, you may as well do the others, why not replace the whole controller board assembly with an offboard GRBL board (it’s just an arduino, then add a GRBL breakout board, here’s how it lays out https://blog.protoneer.co.nz/grbl-arduino-g-code-processor-pin-layout/ ) and separate axis drives for all of them?
On the other hand, replacing the Z axis with a screw drive instead of the belt drive might be a better first step - the belts stretch, etc and will do that a lot more with more weight on the Z axis like that. There a link over here: Z axis Ball Screw Conversion Kit . I’m using this one, as are several others here, and they seem to be working pretty well. I’m not using a different spindle (not yet, anyway), but it’s pretty robust.