I did something of this sort with Unbound and AdGuardHome. I kept Unbound on 9 of my VLANs plus localhost. (10.0 thru 10.8, and localhost) The 10th VLAN (which is streaming TV i.e. Roku and Apple) has AdGuard listening on port 53 and forwarding to localhost:53 for upstream. I did this lazy approach so I could see what the streaming TV's are doing. Also did an outbound NAT port 53 into localhost:53 to stop the Roku going to 8.8.8.8. Next step is looking at ZenArmor to stop DoT & DoH from getting out, as I see my iPhone when on Wifi goes to some dns-apple.com site it looks like for resolution. So far it's working good. The only gotcha, I had to modify my floating rule to reverse/ignore via an alias my domain/dns to allow them outside access (no blocking of anykind) as backup/testing of name resolution.
If you are using port forwarding rules, you should consider to create interface groups in Firewall: Groups. One for Adguard interface(s) and one for the unbound interface(s). For each group a net will be available to use in the Firewall rules, so you can create in Firewall: NAT: Port Forward a Port forward rule with the destination from that group net to Adguard and one for the the other group to unbound.You can also apply firewall rules then in the groups, which will appear in Firewall: Rules: <GroupName>.Keep in mind that this group rules are processed before the interface rules.KH