what’s the best approach for publishers who already have existing handles on those platforms?
That’s up to you. For me, these accounts serve very different purposes. I’m only planning to use my Ghost account to respond to comments on what I post there. My existing accounts fediverse will continue to be used for what I currently use them for. Mastodon for general microposting and open discussion, PeerTube for posting video, etc.
Most software for running fediverse services is intended for community-hosting. Small services, run by the people using them. If the software fetches and stores the full post history of every account someone searches for, the bandwidth and storage costs is affordable only to those running large platforms (as with ATProto Relays like BlueSky’s).
You can also follow your Ghost account using an existing account, and Boost your Ghost posts. That will propagate them across all fediverse services where someone already follows you.
You’re asking about migrating @publication@yourdomain.foo to a different Ghost service, or to a different blog host that also supports ActivityPub, yes? That’s a very good question, one that deserves its own topic.