On Big Sur, M1. MailHub’s Mail extension has an option to prevent Mail from marking mail as Read automatically but they have no version for Big Sur so far. I’ve been scratching my head to see if there is a way to accomplish this is AS but can’t see where to begin. Pointers would be very welcome.
There isn’t one.
SmallCubed Software’s Mail Act-On has an option to change the mark-as-read delay, but you have to buy their whole mail-suite to get it.
I own both MailHub and Mail Act-On and am still working on Mojave at the moment.