I have started just using M. Like, "Dear M. Lastname". I saw it being done in French and just adopted it for English too. No one's complained yet. (Have also seen Mx. but figure that could be confusing.)
Just fyi, M. in French is only equivalent to Mr, it's short for Monsieur. Mme (short for Madame) is equivalent to Mrs, and Mlle (short for Mademoiselle) is equivalent to Miss/Ms.
So using M. for everyone is equivalent to saying Mr. for everyone.
Yeah, I know! That's why I was surprised when someone addressed me as "M." (while I don't present as such, traditionally) in French. So, I thought "that's neat!" and picked it up.
In the sci-fi book Hyperion (which takes place hundreds of years in the future) they use this convention throughout and it works really well, so I've also wished that it were widely adopted in our society. (Except for androids, where the title is A. rather than M.)
I've been playing though Prey and seeing people refer to Morgan Yu as "M. Yu" always throws me for a loop because I think they're using an honorific. Doesn't help that Morgan is already designed to be a gender-ambivalent character so the player can choose their gender and the story will still be the same