One problem, even without all the obvious perfect character stuff, is that if the only way you can write a compelling character is if it’s a stand-in for you, then all of the other characters are going to suffer. Even if the others are not outright badly written, the difference in quality may be obvious to the reader.
Probably not a Mary Sue per se, but I really hated the way Stephen King wrote himself into his Dark Tower story.
Well, if you feel unsure inventing a opposite sex character, then just cast one instead. Find a good actress in the last 75 years who’s done the heavy lifting of inventing a character, and let her move in.
So you’ve read Six Feet Under Par (A Chip Driver Mystery) by Brent Norwalk, then?
I wrote myself into a fanfiction. I’m a background character who got lost trying to find a State Park.
To me, the Mary Sue concept has evolved beyond needing to be an author insert. They’re more of a walking deus ex machina. They’re a new character who is perfect and comes in and resolves things for everyone else. Being an author insert makes it worse, but is not strictly required, since you can judge a work without knowing anything about its author.
I do think you could do this, but it would likely be a comedy, as the concept removes any dramatic tension.
As for what the OP does: you can get into trouble with an author insert being too important to the plot. But that’s not the same as writing characters based on yourself, which every author does. As long as the different characters are different characters, they can even be main characters. This is much harder for an author sureogate, and especially if you have more than one in your body of work. Having them be an ancillary character works a lot better. To avoid Mary Sue claims, it would be good if they don’t come off as too perfect and don’t resolve problems too easily.
In the story that has been suggested as one of the first examples of SciFi, Margareth Cavendish not only has a clear Mary Sue as the protagonist, she then has the protagonist hang out with Margareth Cavendish. It all becomes very meta.
William Goldman is arguably the main character of The Princess Bride, though he goes out of his way to portray himself as more of a sad sack than his real self.
Author surrogates can be good. Mary Sue characters are almost always (except as deliberate parody) bad.
My favorite self-insert fanfiction plot is when they write themselves into the story then deliberately break up a “canon” couple just so their character gets with them instead.
So many non-sex story that still have to bring up the fact the self-insert character has a bigger penis or bigger breasts than the actual canonical characters.
You forgot the raven hair and sapphire blue or pure green eyes of the Mary Sue. And a ridiculous name.