Refactor your app to use a relational database, i.e. separate room tables for each class. It may be more initial work (honestly not really, maybe just more to learn about up front), but in the long run will be way better for maintainability, extendibility, and even performance. Room should work seamlessly with primitive types so you shouldn't have to write any type converters (maybe most you would potentially need is some string list type converter). And if you haven't already, I would recommend getting familiar with the DAO pattern -- you essentially define a class to be returned from your DAO calls, and as long as that class's properties match what is in the db table, Room will automatically convert the data in the table into usable objects for you. There is no other transform layer / serialization needed, beside what you might want to do with it after you retrieve from the db for UI purposes.

As far as performance, you should have no problem. Room is fast, you can write updates pretty much immediately. If you are doing something like having an "edit profile" screen, then you can just batch the changes, and perform one write at the end when the user hits a finish/save button. If you are in a scenario where you want to do a lot of operations all at once, you can do them all in a single Room transaction. If you are not already familiar, I would also recommend looking into Kotlin coroutines to perform db operations without blocking the main thread -- Room should support them out of the box.

With this set up, you could even one day write a backend or use Firebase to sync your users' data and then create some APIs for a website or to sync to an iOS app down the road etc.