I'm interested in this too, Tomer.

The best I've seen for preserving content on Nostr is Nostr.Build. However, under the hood, it uses the Amazon cloud and cloudflare, so that's your failure point, eventually.

I think the answer of posting to many relays, including your own, is a pretty good answer at this point. I don't know if it's perfect, but it seems worth trying. 

So, post in many different places and services. That seems to be a good approach, even if there's no guarantee.