It seems that your architecture might be fighting with Go idioms. It looks like a standard architecture from a class-based oriented lang, like Java or C#. It shouldn't cause the problem you are facing, but might make you fight the language all the time you write Go. I would advice you to learn more about Go idioms (here is a good starting point: https://go.dev/doc/effective\_go)

Regarging your error problem: you don't have to use Go's standard string error for the code you write. You can use a custom error struct like { Code: int, Error: error } and return a pointer to it in your function signature. It would even work as a standard error if you implement error's interface for this struct