The cost you mention for PoW only matters for protecting the history of transactions, it is irrelevant for spam, it actually protects the history of spam as well. PoW works because someone trying to redo transactions would have to have more PoW than the cumulative PoW of all others.

There is no cumulative PoW in nostr, unless you are proposing to turn nostr into a blockchain but that sounds absurd to me.

Maybe I'm missing something so don't let me stop you, if you find a way that works I'll even implement it in Nostur, but I'm not convinced right now.

Just a coincidental anecdote, yesterday I was trying another nostr website and was wondering why the submit button was so slow, when I looked at the produced JSON I understood why when I found a nonce in there 🤷