This is something that coming back over and over again. For most windows admins is/was an issue till the Windows 2k16. I always preferred to handle split-dns in a linux environment than building a separate server just to provide the external, NAT, Geo-location IPs.
Though as the Active Directory loves DNS, it is hard to separate it. At last we do have a solution with the new policies and split-scope zones. Below I will provide a simple scenario where you are having two networks and the subnets are NATed. In my experience the best way to handle it is by scripting and building the server from the scratch. Outsource you can add the AD integrated zones, but there are things you maybe don’t want to inherit.
So at first is good to create a number of CSV files to hold the SRV, A, CNAME , Zones and the outsource don’t forget to add , where is needed, a column with the NATed IP.