Do you have 100,000 clients? If not, the absolute best answer is to Migrate from your less-than-efficient, should-never-have-had-a-CAS hierarchy to a Standalone Primary, and use Sender-capable DPs in the sites where you've put Primaries.
If you DO have 100,000 clients (or more) then I suggest finding a consultant firm which has handled designing and making recommendations for a site of more than 100,000 clients. There are multiple ways to design (or re-design) with a CAS and
primaries, but there are considerations specific to your company which may make a 'random recommendation' on a forum like this leading you to a completely horrible destination--which could make your issue with replication WORSE. So I don't think we want
to go there, in a forum.