If by adding new methods you mean to add new responsibilities to the class then you might consider using the Chain of Responsibility Pattern. https://en.wikipedia.org/wiki/Chain-of-responsibility_pattern#:~:text=In%20object-oriented%20design%2C%20the%20chain-of-responsibility%20pattern%20is%20a,to%20the%20next%20processing%20object%20in%20the%20chain of Responsibility Pattern.
But if adding new methods is simply a matter of updates then you might use partial classes to divide your class into smaller chunks of code for improved readability.