I am sure your project is DO-ABLE but you might hit a few problems with GRBL itself.
It lends more to an increment from zero for most outputs and a set of defined pulses.
Really does sound like a more expensive solution would be where you need to head.
Not saying it cannot be done but I think the pitfalls would be the biggest stumbling block
Only controllers I could find were quite expensive so you would probably have to build your own custom units.
And again its down to cost vs adaptability.
We asked for some motor specs and I think they might help narrow down your needs.
Just saying what they are as you did still leaves some questions unanswered.
BTW you are allowed to use line spacing around here 