I’m a little thin on exactly what you are trying to do here but this seems like an ideal machine for a timing belt drive. Beyond that I suspect that you have this machine under funded.
No matter what your friends are right about driving from both sides of the gantry. This especially with the very loose linear bearing arrangement you have. The bearing solution isn’t bad for what I understand you are doing but they will allow a bit of twisting. Timing belts on both sides of the gantry would solve that to a good enough degree.
However this will not be a trivial timing belt installation. The belts will need support an a good wrap around the drive pinion. We are also talking belts here that are rather wide, maybe 50mm. In any event this is likely the cheapest way to get positive drive for a long distance you need to travel.