: Maybe a standard understern would be the most simple to make and
: less problems than a retractable. Maybe also less weight.

Simplicity and low maintenance are not likely characteristics of combined understern retracting and rudder functions. (I note that even after partial installation below (prob costing close to a grand), one person was persuaded to refit a basic skeg because of size, complexity, and lack of reliability.)

As the above 2 characteristics are important to you, maybe go with the std understern and maybe make the blade into a very simple breakaway with a bungee tether (possibly internal like a tentpole) - so no loss, no damage to the main structure on impacts, and a simple snap back on by your buddies, or as you remount - or maybe just a pull on the bungee.