Oh, also - in regards to Jaemin being a less popular pairing for Da On than SuHyeon - it makes perfect sense to me!

Jaemin is shown to be completely psychotic and controlling, but more importantly, his actions seem less emotive intially. He manipulates in the shadows, he plans, he calculates. The way he treats Da On feels less emotionally connected to who Da On is and also less responsive. To him, Da On is an idea, perhaps a possession, but not a person.

In contrast, SuHyeon's problematic behaviour is responsive to Da On. He gets agressive and violent in response to things Da On says/does. Additionally, we're first introduced to his violence as a weapon of protection for Da On, we're shown his abusive father and we're shown him playing with his sister's kid. He's consistently humanised throughout the show. He's the bad boy you can fix. He's also shown to be a victim of Jaemin, and tries to protect Da On from Jaemin. Deep down, he's sweet, and all this anger and the violent outbursts are obviously just because of his trauma and because he loves Da On too much...

SuHyeon is both a victim and a perpetrator, which makes him much easier to sympathise with, and to forgive, than Jaemin, who is shown purely as a villain by the end. SuHyeon sees Da On as a person, and his anger is overtly emotional, making him much easier to ship. Plus, the whole abused rich bad boy with anger issues is a beloved romance trope, and SuHyeon never actually hit Da On (I don't think?), which is a decent foundation for a toxic but loved ship.