own fields without "Custom Fields"

here "https://akaunting.com/docs/developer-manual/overriding-output"
and here too "https://akaunting.com/docs/developer-manual/hooking-models"
is roughly described how to e.g. CoreModule could customize.

But how should one proceed if one without a "Custom Fields" e.g. Would like to have the first name field with validation at the customer and also want to adapt the blade?

You can't do any validation via observer and copying is also difficult.

Do you have to overwrite a CoreController or CoreModel with your own module?

Did you ever work out how to do this?

No, unfortunately not