What I do with my seawater flush toilets is to fill the bowl up with fresh, then pump it out. That ensures the only sea water is in the feed pipes and as they mostly self empty on haul out. They are also mostly under the floor or in the bilge and actually never smell. I'm wondering if I really need to complicate the installation (and to increase my water use).
Obviously if that method works for you, keep doing that. I'm always on seawater, never turn off anything, will visit whenever and use the head, so not easy to organise that tbh.
when I fitted a second Tecma, it was plumbed with the option for either fresh, used most of the time, or seawater when required for longer periods away or multiple users aboard, then do final flushes with fresh at the end of the cruise.
A switch controls which water type is flushing, a Marco pump draws seawater and a valve prevents seawater leeching back into the fresh water supply. Simple to fit and been operational for four years so far.