Use Fermcap so you don't plug the QD.
You could also just keep the keg open to reduce risk of overpressure. Either keep the valve open, remove the lid oring, leave the lid on but loose, etc.
If you want to cap it for natural carbonation, just wait until fermentation starts to slow. You'll probably end up overcarbonated. Up to you if that matters.
If I'm using smooth puree or juice, I add it in the keg. No need for secondary. Fine after 2nd fermentation is finished then dump a pint or so of yeast.