First, perhaps understand the Apple Support Community is a user-to-user technical forum. Contributors here are all end-users, just like you. Other than the site Moderators, Apple neither monitor nor participate here - and as such your commentary here will not be seen or acted upon by Apple.
Apple do invite constructive feedback, comments and feature requests via their product feedback pages. For iPad and iPadOS:
Feedback - iPad - Apple
You are encouraged to make use of this facility in expressing any views concerning issues or functionality changes of which you are unhappy.
Retuning to your observations concerning the Notes App, all edits are automatically saved as you edit - making the need for a Done or Save button redundant. This feature does have useful benefit in ensuring that the Note or subsequent edits are not lost if inadvertently closed prior to manually "saving" changes.
Unfortunately, there is no available method to lock a note from editing without making the note inaccessible for viewing. The available function to lock a note prevents all access until unlocked - at which time edits are permitted. You might consider submitting a feature request outlining your need for a simple reading-mode via the feedback portal.
Given the limitations of the native Notes App, given your stated need, you might consider using an alternative third-party note-taking App from the Apple App Store. There are many from which to choose, which may better meet your needs.
