but good luck getting your whole family, including elderly relatives, to use something you do control...

That’s one of the things I like about NextCloud. Even the most non-technical person knows how to follow a link in a email (to many an IT tech’s lament). All I do is share the file in NextCloud, maybe password protect it with a simple password, and copy the share link to the email or text message. Bob’s your uncle. Grandma Nosy-Britches gets to see the files but her email or messaging provider (Google, Mircrosoft, or whoever) does not…. At least until she shares the file directly. More likely though she will share the link. But that’s probably not something I’m too concerned about.

You can share pics by a secure channel and they'll just repost it all over the place anyway.

“A secret shared is no secret.” If you have a problem with something being shared, don’t share it. With anyone.

I’m more concerned about accidentally tripping safety or security systems I don’t fully understand and having something I depend on suddenly cut off than I am the vagaries of dear Aunt Noisy. I can pretty well guess what Aunt Noisy or Grandma Nosy-Britches will do and it’s either not a problem or I’ve taken steps to avoid any problems.