by portraying one accent phonetically, you implicitly suggest that the speech of other characters in the book is "unaccented", or "normal".

Pretty much. And the rest of it is also on point.

I'll add that there's one particular (French) accent that I cannot hear (ie the people who has that accent literally sound the same to my ears as people who have my accent) so that would get very confusing very fast if anyone tried to write that accent out.