Thank you, @theluthier, for the instructions on how to solve this problem. I was already anticipating that this could be necessary since using “Symmetrize +X to -X” ended in this:

I promise that I will never do this again with poor Piero ✋! And in the meantime it’s already good practice to create shapekeys 😉. My goal is to make Piero smile 😊. After this it’s time to tailor him a beautifull feathering 😀.