Especially for those of us with no coding background, who can’t just build/edit an app.
For example, I like using Gadgetbridge to replace my fitness tracker app: https://gadgetbridge.org/
It’s not nearly as full-featured as a regular fitness app, but it literally can’t connect to the internet so there’s no way anyone can get my health info.
How do you feel about using Syncthing? I use it to sync my notes and files between myself, but it could work in a family setting.