Since there are no dimensions on this plan, this is conjecture, only. But I would simply remove the shower and tub, extend the vanity to where the shower currently is, then put the larger shower opposite the vanity. Remove the swing door at the bath entry and replace it with a pocket door so you have a bit more room. Shower needs to be at least 3' wide to be comfortable. Leave the toilet where it is.

I agree that would be the best without changing any major plumbing issues. Remove the door off the toilet room , no one needs to use the toilet while someone else is having a shower and replace the other entry with a pocket door as mentioned .