I haven't used it so I can't recommend it, but there is a
COTS all-gear 6 wheel drive available from AndyMark. As noted above, it's heavy and expensive. If you want to go down this road, you may want to use this or just examine it for ideas/inspiration.
Edit: We have used nanodrive, just not the all-gear. Kind of strange to assemble, and the gearbox is not fully enclosed, but our air cannon (which is quite a bit heavier than FRC legal) runs well on it.