I've struggled with this for years, and the fact that there are dozens of similar queries and responses suggest that creating the function ought to be a priority for Instructure. Shar's very solid 2020 summary laying out different extra-credit options can be found in this response.
Oddly, it seems like a comparatively easy coding design if someone managing development would just prioritize it. Quizzes and assignments already have a checkbox in New Quizzes and Assignments for not including a submission in the final grade (a function I use to create schedule reminders for students). It should be simplicity itself to create a new check box to mark New Quizzes submission or Assignment as extra credit. The challenge is how those scores would be calculated into a final grade. One simple option would be to create a checkbox in Settings to turn extra credit off or on for the course, to toggle/reveal a Gradebook column for a final grade plus the scores submitted which are marked as extra credit. Just give the column background a different color to distinguish it from the required-submissions grade total. That would leave the current coding for the final course grade unaffected but give instructors an option for summing extra credit and reflecting the adjusted score against the required submissions. Just sayin'.