I iterated on it 3 times until I settled with a solution that doesn't rely on Google/Apple/Microsoft for syncing. In short, this means that these companies can't access the nostr keys.
@daniele you could see the nostr-based sync in action by following these steps: 1) visit
https://44billion.net 2) Click on the account avatar and access the signer (aka vault) 3) Click on the "Sync" button and follow instructions. Do that on two devices or two different browsers.