While I also struggle with the android/apple divide in my family I feel like getting them all to agree to use an existing social platform would be exceptionally difficult, but it would be orders of magnitude easier than getting them to use a custom or relatively unknown app for comms unless it did something really special or novel.
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?
The pain point for me is that my family/friends circle isn't willing to try and use new software especially stuff that might be custom or have some lack of features.
It took me a year or more to get my dad to send me pictures and videos over FB Messenger instead of SMS (he has an iphone and I have an android). Maybe I just need some new persuasion tactics but the idea of putting energy and time into building a custom app for my family and then dealing with pushback and a reluctance to use it just sounds exhausting to me.
Yes, I have toyed with this idea, it is after all almost exactly what I'm looking for. But ultimately rejected it for the reasons @horsawlarway stated. I guess I just don't trust Reddit.