I just put together the BOM for the belt system as an alternative to the ballscrew and in terms of cost saved... there's no cost saved.

It also seems that not a lot of holding torque is actually needed for a 1605 ballscrew:

Even less for a 1204:

Which means that the 14:1 reducer motors can be removed too in favour of the £4.80 motors used in the rest of the printer.

Then there's the risk that if the top pulley assemby breaks, the resin cartridges are going to fall down like underengineered lifts in action movies.

Overall, it seems that belts would end up being more expensive than ballscrews.

[16:32] Confirmed: