How much tweaking of figure design (not content) can I postpone until the final figures are requested?
As a referee, if figures are in a rough shape, I will absolutely call the paper out on that, and demand improvements. This will potentially set the acceptance back by weeks. Unless the figures will take weeks to get right—and in this day, that would usually imply  that more data needs to be collected—then it's better to just do it right the first time.