Or with build-time source generation (because this specific pattern of reflection is AOT-unfriendly). It's not as convenient if you are using default serializer options, but if you don't - it ties together JsonTypeInfo<T> and JsonSerializerOptions, so it ends up being a slightly terser way to write it. I do prefer the Rust-style serde annotations however.
record User(string Name, DateOnly DoB);
[JsonSerializable(typeof(User))]
partial class SerializerContext: JsonSerializerContext;
...
var user = new User("John", new(1984, 1, 1));
var response = await http.PostAsJsonAsync(
url, user, SerializerContext.Default.User);