All of this is just using the base syncthing and doesn't require any technical skills. Backing up in the cloud needs more time and expertise, but after having done it myself and realizing I don't actually need it (a simple backup is enough, I don't need a web interface to do what I can do faster and more easily from a desktop app) and I can have encrypted backups instead. Bonus: if you're both on the same wifi the sharing is quasi-instantaneous and doesn't waste energy sending to a faraway server first
I have shared albums with my wife that are generated automatically from photos of our kids based on facial recognition, and then we have a shared album with family where we add curated photos to - as the initial album is shared, we can do this with the other person's photos, not just ours. This works pretty much seamlessly for both photos and videos.
I recently migrated my 100K+ photo library from Google Photos to Apple Photos, and I can’t think of anything I miss.
I suspect Google Photos may be better for setting up a shared folder for a bunch of random people at an event, but I also haven’t tried that with Apple Photos.
For me the key feature is partner sharing. My wife and I take lots of baby photos. We see each other's photos as if we took them. This is the critical feature, and there is I think a large market for it. Even the slick ability to browse photos on the phone is nice but non-essential. Sync two people's photos reliably to a common S3 bucket, and even a janky web interface (+API) will suffice to get us to migrate. Convincing just one partner is easy.