[smush]

I agree with you 100% and will amplify your comment with one of my own.

I suspect this won't be immediately possible as I'm not certain what languages OP is allowed to use, but if OP has some wiggle room in what PL to use, then I would recommend that he look at the Domain Modelling Made Functional book by Scott W.

Like you say, the principles in the book apply to other languages as well, but it can be a faster feedback cycle to learn these lessons in a language that has first-class support for them. He will be able to use F# or its close syntax cousins OCaml, ML to model new features as domains and employ option types, pattern matching, railway-oriented-programming, and functions as interfaces to make his code more resilient, like you mention.

The bonus is that if OP uses the likes of F# or Scala, it can interop with C# or Java respectively within the same codebase.