The Natural Philosopher wrote:

Filling and sanding are hours per bay.
That's what I want to throw money at to get the time down
So far it seems that there are lots of suggestions but not many to
actually achieve what I want.

I think you should be able to reduce the sanding time, and make it
hand-sanding rather than hoisting a machine over your head.

That probably comes at the expense of slower, more careful filling, and
time between two layers of filler, but as you've plenty of bays to do,
you probably have no need to actually waste time between layers, just
work your way round and then back to the start.