The only problem is that it will be considerably more expensive for the tradesmen to stop and start leaving the site safe and habitable each time.

This doesn't seem like it would really take that long to do--the longest part being the tiling.
Since you aren't tiling the floor, is there a reason you think it needs to be done in stages?

I was thinking we'd always have access to something in the bathroom? For example, even if the tub was still being worked on, we could sponge ourselves off at the vanity and vice versa?

Now that I think about this, it's probably wishful thinking. :/

Well, you still have the kitchen sink, no matter what, right?
It seems to me, I'd be most concerned with Toilet utility during a remodel, since this is your only bathroom.

So, I'd talk with my contractor about how to make sure my toilet was mostly always usable.

Other than that, make sure your fixtures/flooring/vanity are all on site before demolition begins, and I think it should go rather quickly.