How To Properly Setup DNS Forwarders On Windows Server

You shouldn't have a reason to forward to other AD servers, as DNS should be replicating between them already.

The only reason I can think of for this would be if you wanted only one DNS server to be the one querying the internet for non-authoritative FQDNs.

Usually the forwarders (used to resolve non-authoritative lookups) are set to external DNS servers like your ISP's or a common public resolver like 8.8.8.8 or just use Root Hint servers.

Conditional forwarding is different though...

Not knowing how your environment is set up and what your goals are I'll risk suggesting that you shouldn't have your DC/DNS servers using any other DC/DNS servers as forwarders.

The only scenario where I've seen internal DNS servers using other internal DNS servers as forwarders is when the security policy restricts outbound DNS traffic to a few secured DNS servers. All other DNS servers forward non-authoritative queries to these secured DNS servers. I'm making an assumption that this is not the case in your scenario.

If that's true then remove the DC's from the forwarders tab and use the root hints or add forwarders to external DNS servers (8.8.8.8, etc.) for non-authoritative queries.