Adding a low-restriction filter to the return will not hurt anything, but it should be located BELOW the reservoir. The tiniest air leak (even if it isn't leaking fluid) can cause the additional fluid volume to drainback and overflow. The benefits of a return line filter is minimal, IMO. Power steering system issues usually involve pump failure, which will cause issues/failures with downstream components. A high pressure filter should be installed at the pump outlet to maximize system protection. Ideally, both would provide the maximum benefit, along with a cooler (which should also be installed below the reservoir level).

I do not recommend using synthetic ATF in place of OEM recommended power steering fluid. My reason for discussing it is purely anecdotal personal experience. I use it, and have had excellent results. My power steering system issues were reduced by more than half when traditional/recommended fluid is flushed and replaced early (a year or less in a new or repaired system). This is my experience with more than a dozen vehicles over 30 years, with and without using synthetic ATF, but also with coolers installed on most of the trucks. I've used only Amsoil and Mobil 1 synthetics, so can't comment on any of the others.