Thanks a lot for the detailed response! I have a few more questions:

1. Will swapping out the gearboxes lower the torque at the wheels? Will this mean that my kid can't drive over the grass?
2. I think my wife would be quite upset if I disabled the remote functionality. If I wanted to take my chances with the board, would I just use an sla in series? Would I simply splice wire of a certain gauge using electrical tape or screw-on connectors to wire up the batteries in series?
3. Same question with the fuse: just chop wires and splice it in?
4. What's a good place to buy an LVC, inexpensive drill batteries, relays and boards? MLtoys seems quite expensive, and so I was wondering if cheap stuff from amazon would work as well.
5. Do you have any experience with this? Would this work as a drop in replacement for my 12V board? This way, all I would need is an extra 12V sla, wire it in series and be done with it!