Ok

What about making it autocorrect irrespectably of what's before or after the 'aa' ?

Is there any character that acts as "wildcard" - i.e. meaning "could be any letter". If for example $ was the wildcard character, then having $aa$ would mean no matter what you write in front of or after aa it would still become ā.

So writing for example Maac would become Māc, 1aa would become 1ā or aad would become ād.

How can you do that?