The doors on the Niro open far wider than most cars so that is not an issue. Rear seats are split so I'm thinking the easiest path for an older dog is to have one rear seat down, and offer the dog entry on the other side. Sort of a two step into the rear compartment with a larger first step. After that, your rear seat passenger can sit down, and depending on human or dog preferences or needs, the other rear seat could then be raised to restrict dog movement or add another rear seat passenger.
2018 Kia LX HEV Metal Stream with Advanced Tech