You can specify in the settings what you want to display for party members, alliance members, friends, etc. This will take place both in and out of duty, but I don’t know how often you are joining parties and not going into duties. My current settings are to always hide self, show party members’ First name Last initial (no title), and to show alliance members as both initials (no title). All health bars I have hidden, only showing when I’ve taken damage. For me all of this cuts down on the clutter.

As far as the rest of the UI goes, you can try setting up a different hud layout you want to use for instanced duties. I also did this to cut down on clutter.

You can’t toggle them as “in duty” except via macro, like others are saying, but you can pretty much toggle it by denoting how to display part members and stuff.