Plumbing fixtures starve but water pressure 50 psi
does the PRV keep a constant 50psi even under demand?

It is designed to try to, by opening all the way up. If for some reason it is not opening all the way up, it could be a cause of downstream flow problems. I would replace/repair that first unless you suspect something else as the cause. Any other restriction, including limitations present due to system design (pipe/fitting size), and/or system condition could also cause the problems you describe.