Run a local relay (Citrine) and it will record a copy of all your notes and replies. Or, subscribe to nostr.wine and their cellar relay which records all your notes and archives them forever for a cost of 1 sat per note per year.
Discussion
I would avoid the counterparty risk of nostr.wine cellar when you can easily self host for <1sat/note. Mazin is a good dude, but no one is more trustworthy than yourself, and no digital skillset is more valuable than personal sovereignty
My notes are public anyway. If I already had a Start9 server, I would run the RS relay server and manage it myself. But since I don't, it's easier and more cost effective to just backup to Citrine and cellar. Between the two, I've got redundant backup for a few thousand sats per year.