Which platform/client/offers user backs ups?  without running a Nostr relay?  ( almost nobody does this because  its impracticable - in the end we just have to trust  that major relay operators will not delete or censor us)

Yes, keys/crypto is an advantage.  I like that about Nostr.   But this does not address the censorship or centralization of relay operators problem.  Nostr doesn't offer what was promised re censorship because  few will run their own relay  and few will use small relays if nobody else is using them.  So if the main relays censor you, it becomes irrelevent that you can use a small unknown relay that won't. As nobody is listening there.