I have a couple of problems with outsourcing this kind of thing like that.
Namely - a private subreddit would work but comes with a LOT of downsides.
- Ads (this is pretty much a dealbreaker on it's own...)
- Signup
- Changing UI/UX
- Lack of control over uploads/content (who backs it up, when, how?)
- Lack of control over the general experience
The nice thing about custom software is that as long as the user count is low, and it's family that you trust, you can ditch a LOT of the pain points of online software.
ex: who needs signup/login when I can just embed a shared key?