I expected something more thought out under the title of a “fully personalized UI”.

Have you actually designed and maintained an app? Personalization is extremely hard to do right. You can’t just throw a 1000 options list at the user and let him construct the app however they wanted. They would turn away annoyed.

If you want to have a personalizable app, you’d need to find ways to slightly alter the mental model of your app to fit the thinking of people coming from different backgrounds. Not just superficial things like dark mode. Everyone wants dark mode, that’s not personalization. It’s a trend. Four years ago people hated dark mode.