Not knowing the “can’t be changed” (could those days be shuffled? or just those are the only possible days as in days of week?)
If you want to work more speed, and given the above disclaimer…dump the 150-250 hills on Saturday and then move the Monday weights to Saturday (as you suggested was one of the choices) and you could also make the Sunday 20 minute a “fartlek” style of run so as to give you short bursts of speed work. Personally I would likely shuffle the days into a different order (comment made with just a scan of the overall, plus more info/detail, say percentages would be needed)