I would only add a single SFF so that I can fit a couple of big 3.5 disks for my backup and data hoarding needs.

Other than that, yeah.... Micro/tiny/micro is the way

If I were to set up a little server for him with docker services running things like immich and syncthing etc, would this be set and forget enough to not cause problems for him? Ideally he wants his own cloud so relatives can store and share files, photos, and possibly movies too. He isn't the most tech savvy, but he knows his way around Windows and PC's generally

Maintain his own cloud without effort or knowledge? And provide it as a service to his relatives? Over the Internet I suppose?

That sounds like a recipe for disaster. In so many levels...

I'd say I am 95% homelaber and 5% selfhoster. Most of my stuff is for experimentation and learning. And most of my services are vanilla ones, like samba. So in essence I am self-hosting not much more than a few linux environments.

The things that are indispensable to me are samba, my docker development stack, uptime kuma, and a simple wordpress installation that I use for notes and documentation. Oh and lately Stirling-PDF. That thing is just awesome.

I have tried various tools, but I keep coming back to vanilla samba for most stuff. Like paperless-ngx. For my needs, it's just a fancy way to tag documents. I don't need full text search or OCR, and I can find most of my files quickly using a simple directory hierarchy. I do not really need the extra overhead of maintaining paperless-ngx. The same for things like Immich, plex or Owncloud. Samba and file explorer preview works perfectly for me.