AFAIK ActivityPub uses a fan-out to other servers using the sharedInbox of the receivers [1], this makes the cost of distribution to be diluted by all instances of the fedverse that follow that account.

And what are you missing on the current aggregator features? do you have in mind anything more then the normal action of following other users?

[1] https://www.w3.org/TR/activitypub/#shared-inbox-delivery