I welcome these but for each of 8, 9, 10, 11 and 12 I still feel that the elphant (missing) in the room is a proper sum type. How come I have to write some 100 line monstrosity of a utility class each time i want "Either a result or an error" or similar.

Discriminated Unions will make working with Result easier as would provide exhaustive checking. DUs are being designed: https://github.com/dotnet/csharplang/issues/113

Fantastic. I note it's indeed being designed since 2017 so it's probably not as easy as I would hope. The design seems to not have progressed beyond discussion at all in 6 years.

Until that feature lands there's this https://github.com/mcintyre321/OneOf which is pretty nice.