You can’t build a box that will survive long without your help. You’re maintaining a living system, not a sculpture. It needs someone at the wheel making decisions. Updates will have breaking changes. Tokens and certificates will expire. Eventually hardware will fail.
The best you can do is provide an easy way to export the important data into a digestible format for your loved ones to manage with the skills they have. If that means pushing it into a managed service owned by Big Tech, so be it. You don’t want to tacitly hurt them for their lack of interest in self-hosting.
Try to use open source software. Harder for it to disappear.