The oauth flow you just mentioned exists, I just did it to confirm my domain in cloudflare with google workspace: google did an oauth flow and I got a cloudflare popup asking me to add a dns record for an hour. It was very cool.
https://www.domainconnect.org/
Don't you hate it when you have a good idea and someone already did it, but also love it because it's validation of your good idea?
[1] https://www.godaddy.com/engineering/2019/04/25/domain-connec...
[2] https://dash.cloudflare.com/domainconnect/v2/domainTemplates...
And yeah I hear what you're saying about ideas haha.