You can still customize your phone. You just cannot roll back an update. You can change the way it looks back to the way you want it for the most part after an update (if your update changes the look). Updates are not optional, and you agreed to any changes in advance by checking the "agree" box on the TOS and EULA when you activated your phone.
Honestly, if you hate your Samsung device so much and you don't want a secure system, trade in or sell your phone and go buy one of the Chinese phones. You won't get OneUI, and the ability to customize and you still won't be able to roll back Android versions, but you likely can update optionally. Or, go buy a used S20, where you will not get any further updates to Android or OneUI.
You can customize or make your own theme by downloading Samsung's Good Lock/Theme Park. You can even use theme park to put the look back that you are crying about. If you need to know how to change other settings back, just ask properly and be specific in what you are asking.
