The best peace of mind is to run your own instance, but it takes a bit of tech know how. But then at least you know it will never shut down. :)

Alternatively, sign up for smaller instances that you think will remain. There is no reason to use the most used instance when it's a distributed system.

You could host your own Lemmy instance, I think that would do what you want.

The thing I'm seeing issue with is the expense of a cloud server and domain just for a single instance. If the possibility were made available to connect a raspberry pi to my network and offload my posts/images/videos/profile onto my own network by secure proxy, I'd do it, or maybe just some kind of mirror that I could port to another instance if the original goes down.