I have been experiencing a number of issues since migrating from iPhoto to Photos, and I would like to provide useful input/feedback to the Apple software designers.
What would the best way to do that?
Apple does provide a feedback form where you can submit suggestions/complaints/ideas.
You won’t get a response, and there are no guarantees they will implement your ideas, but I’ve seen good evidence on a number of occasions that Apple does take such feedback seriously.
When voicing your concerns just remember that respectful discourse is far more likely to make an impact.