Low-maintenance split horizon DNS?

Currently several of our main domain names are manually replicated into our corporate windows DNS servers from our api-less DNS provider.

As you can imagine, over the years we’ve got loads of discrepancies between both zones, records existing on one side but not the other which is pain.

I’ve started to shift domains to existing externally only and removing our internal zones where possible, which has some success, but the exception to this is where we actually want record asymmetries, these are where we have private routes to various services we’d rather use internally.

I’ve actually mostly dealt with this issue via planning to keep only a single zone internally, companyname.com. On this zone I’ve created publicipname.split.companyname.com which exists both internally and externally. Externally it points to the public IP, internally it points to the corresponding internal routed IP.

So when I have a website I want to use different routes I create a CNAME for www.websitename.com to publicipname.split.companyname.com which then handles the split routing without me having to host the www.websitename.com zone internally.

The problem is that I’m left not knowing how to route websitename.com because of how you can’t CNAME the root record.

How should I fix this issue?