I did watched handmaiden but not entirely, but I remember reading fingersmith written by Sarah waters. Handmaiden…
So you missed amazing drama for fifteen seconds sex scene (The Glory) okay, lol... and yes, it isn't really out of place, as the affair is important part of the story, also adults have sex, and it wasn't even naked...
If there is not a typo, it would make them 31-33 in the present, which wouldn't be that bad. I think hairstyle and clothes would make him look older and it's not like there aren't men who look decade younger, so...
I just don't believe anyone can ship Shun and Dai at this point so far, unless it's because they're young and "cute" and there is height difference... My opinion.
while it's important to be attracted to each other, it's clear that Dai is easily swayed by a handsome guy and looks are super important to him, which doesn't make him a bad person and it doesn't mean he's a cheater of course, but for me, it kinda makes him a red flag and he and Shun wouldn't work... I would be extremely shocked in bad way, if they ended up together...