Twenty-four hours or so of testing and researching, and the one thing a photo-sharing site needs turns out to be missing. Sharing with the internet.
Pixelfed is reasonably slick. It uses ActivityPub to share postings with the rest of the internet, it has evaporative 'stories', it presents images, galleries, and videos in a slick package.
It has no way for people who are not logged in to view the media being shared.
This does not meet our needs, so I have shut down the experiment.