Your appsettings.json looks good.
The sign-in redirect URI should be this:
https://ud-dev.domain.com/authorization-code/callback
That’s formulated by concating the PSU URL with the CallbackPath:
"CallbackPath": "/authorization-code/callback",
I’m not aware of any configuration for the initiate login URI. The steps in the document were exactly what I took to get Okta working for me.