There is no such thing as an app that can offer “whole-disk” encryption on an iPad since no app can alter or access any other app’s file area. iOS and iPadOS are “sandboxed” systems where apps are only allowed access to their own file silo apace and cannot access any other app’s silo.
Plus, the operating system itself encrypts all your data on your iPad Pro already. Nobody can de-crypt and access your data without knowing your device specific screen lock passcode.
The kind of app your talking about only provides an app specific passcode as well - any app can offer and use an app-specific passcode or password but it is up to each app developer to include that feature. But other than an app specific passcode, they don’t add any additional security since even their data is already encrypted by iOS/iPadOS.
There are any number of vault or secure storage apps available in the App Store. Beware of ones where the developer is a single person or a largely unknown developer. Such apps have a notorious history in the App Store of being suddenly dropped by their developers when there are major iOS or iPadOS changes (e.g. a number of photo “vault” apps were abandoned by their developers when the switch from a 32 bit to a 64 bit system was implemented and many users found it was thus impossible to access their locked photos after they upgraded the operating system).