See, I think Mary Bennett could absolutely be hero material, it's just that she couldn't be the hero of Pride and Prejudice. Her story has its own themes to play with.
I think that Mary has fallen victim to the Classic Autism Blunder- if someone tells you "this is what you need to be successful! Here is a list of concrete skills and talents you can acquire! Here's a bunch of frustratingly nebulous social skills you need to cultivate!", and you're on the spectrum, it's really easy to go "cool! I will do the concrete stuff that I know I can learn how to do and ignore all the boring difficult social graces!" Except that, outside some very very rare circumstances, the horrible social bullshit is the part that is going to make or break whether you can be successful.
So Mary is working very very hard to Be Accomplished- the piano! the moralizing! The reading! - and to show off that she's Accomplished. But she's presumably largely self-taught (you think Mr. Bennett is paying for tutors? Hah.), and so is not as good at any of this as she thinks she is. When you factor in that she's lacking in social graces to boot, she's embarrassing herself. But she doesn't have the social skills to even notice that this is what is happening, much less understand why she needs to notice.
If I were going to write her, I'd probably have her be able to realize that people don't really seem to like her or to appreciate her accomplishments... and have no earthly idea why, because no one has been able to get through to her why what she's doing is wrong. She knows that people react really badly to her when she plays the piano without being asked or when she tries to show off her Deep and Profound Learning. But she's under the very mistaken belief that she just isn't trying hard enough, and if she can just impress them enough with her Accomplishments (TM) then maybe, maybe, people will start to like her.
(And, tbh, I don't think anyone has tried particularly hard to explain it to her. Jane is too kind to point out someone else's flaws, Elizabeth finds Mary super grating and interacts with her as little as possible, Mrs. Bennett doesn't care because it doesn't seem to get in the way of her Look How Accomplished My Daughter Is Please Put A Ring On Her quest, and Mr. Bennett encourages her to be Like That because he thinks it's super funny.)
And the problem for Mary is... I've said it before, I'll say it again- most modern fiction set in the Regency* is functionally fantasy, it's just set in a world where social graces are magic. If you're writing A Bridgerton, everyone wants to see the heroine destroy her rival or the local Lady Catherine equivalent with three carefully chosen words and a spockbrow, without being visibly impolite. They want to see the hero and heroine verbally duking it out as a way of carefully testing each other's boundaries. Dances are wizard duels. The drawing room is a battleground.
If you look at it through that lens, Mary Bennett is actually a pretty common hero archetype- she's the kid with no talent for magic who is, functionally, trapped at wizardy death school. If you're a woman in a Regency romance, and you don't get married, you're boned, and everyone knows it. But Mary does not really have the skills to come out of The Season with a man on her arm, and everyone knows that, too. So Mary has to use whatever other skills she has- some of which will still get her a 'good grade in Accomplished Lady', some of which 'wizardy' society has no interest in- to get by.
I think you could get a really good story out of... say, a German princeling coming to town, who happens to be the most neurodivergent man in the three nearest continents, who thinks Mary's flavour of doggedly persistent autism is deeply charming. He appreciates that she can speak his language- in both senses- even if her grammar isn't quite up to snuff. He's well-read in like six different languages, he's interested in science and art and morals and would like a woman who's actually interested in debating all of these things, he's incredibly sick of the petty social-wizardy death game that is the marriage market, and he'd very much like to find a wife this year and be done with it.
Mary has to deal with her mom shoving her at him by any means necessary in ways that even she can realize are profoundly embarrassing. And with the fact that, at first, she kind of hates this guy, because he's willing to do things like 'play devil's advocate for atheism to make you really defend why religion is important', which she sees as Absolutely Heinous and Rascally Behaviour. But because of her mom's clumsy social maneuvering, they're always kind of stuck in the same room together, and he's the only person who isn't constantly reacting to her like she's doing something wrong for reasons she can't ascertain...
...and she's slowly realizing, through arguing with the princeling, that she's neither as well-read nor as deep of a thinker as she thought she was, and forcing herself to grow out of her Tumblrina Your Fave Is Problematic phase into having actual well-reasoned moral convictions of her own and being able to defend them...
...while making a fool of herself, once she's decided that courting him is necessary, by trying desperately to impress him with her other Accomplishments (TM), which would not be particularly impressive to him even if she was as good as she thinks she is, and she is not nearly there....
...and realizing that, no, this man is not a rascal, he's also got strong moral convictions and is also willing to defend them, and they're actually perfectly suited for each other if she can catch up to him intellectually.
It'd be about Mary figuring out how to woo and live with this man in a way that actually works for the both of them, rather than trying to follow her broken-ass social scripts that she can't really pull off. About her finally dropping the idea of Being Accomplished Being The Way To Find Love And Respect. About both of them figuring out how to handle the social obligations that come with being a Goddamn Rich Person without going insane, and having an easier time of it when they've got someone else to lean on.
I think that's an incredibly compelling story and one where Mary is absolutely the heroine, no character assassination needed.
*I don't think this is necessarily true of Austen, but any modern Austen adaptation is probably going to be a Regency Romance (TM) even if its source is not.
