We do have a function similar to what you’re suggesting in the Raid Finder. Granted, it’s very limited since choosing to be paired into a “Practice” run only lets you choose between “First phase” or “Final phase”.

If it were to be implemented, the function already exists — just needs to be reworked a bit to work for the Duty Finder. Though, there is a chance that implementing such an option may spike queue times for players with certain playstyles, which is something the devs probably don’t really want to happen.