Hey @will.qiu!
Thanks for the thorough response. While I was waiting I poked around the forum more and stumbled on this post.
It led me to try disabling AstroWarp Cloud VPN. Once I did that, AdGuard Home accepted the Control D dns server without issue.
Seems like there's some conflicting setup there.
Everything is working as I want now, so I won't attempt the instructions you provided but I'm certain someone else will find them useful. Thanks again!