Is it possible to specify JsonSerializerOptions to use with the Pulumi.Config.Get/Require methods or would I need to specify it manually as an attribute for each property in my model?

Unfortunately I don't think that is currently possible. You could open an issue and state your case if you think it should be, but also remember that there is nothing stopping you from deserializing the Config file on your own.

I recommend using dotnet config sources instead