That's essentially what this system does — it identifies the models and their properties that you've marked as serializable at build-time using source generatio

That's essentially what this system does — it identifies the models and their properties that you've marked as serializable at build-time using source generation, and then allows you to provide a type resolver and converter to System.Text.Json that lets you make upgrade-able models with logic like you just described. The assist from the source generation helps reduce some of the boilerplate you need, but there's no escaping it ultimately.