There’s a big difference between a toxic RO I enjoy and one I don’t enjoy, and I guess that’s because of the ways those toxic traits are expressed! (Or which ones on the long list are selected, tbh.)
Ones I don’t enjoy reading are typically uninterested in the MC, in the sense that while they might be attracted to them on paper, it isn’t something you feel or really see on the page. There’s little affection between the RO and the MC, no devotion at all, and their likely bad treatment of the MC is sort of… Hoisted on to them with little or no explanations.
A character who mistreats the MC for something out of their control, who shows little remorse for it (or never acts upon it) and who nobody ever steps into stop or defend the MC against isn’t enjoyable to me as a romance option, even if I might come to like them as a character outside of it. And I’m a big fan of the classic “tsundere” archetype in otome, but I guess that might be because they usually show their soft spots much more often?
I can enjoy a degree of downright verbal abuse from RO in fiction, but the limit for me is if they never feel a shred of guilt for it or cannot be argued against. It’s miserable wet cat red flag x semi-normal guy or red flag x red flag that does it for me, not red flag x a poor guy who should consider couple’s therapy stat.
The ones I do enjoy reading are typically more on the “so horrifically in love with the MC that their morals start to get a little wonky (or preferably, were wonky even before, and are now getting worse)” side. Jealousy, obsessiveness, and possessive tendencies are completely fine, but even better if they feel bad about it and have to “reason” with themself to not feel guilty.
If the RO is somewhat dependent on the MC and the MC is also somewhat dependent on them in return, that’s perfect for me. It’s just two mutually deranged people tackling the world together, or taking each other down because they don’t want to ever have to live without the other.
(On a side note, I think this is why I especially like toxic queer stories. Most of them are written in a way that adds a layer of secrecy and fear to bonds that often makes them stronger yet more volatile. When someone is the only one in a judgmental, possibly dangerous town who can understand why your idea of love is the way it is, and therefore becomes the only one you can trust, no matter how bad they may be for you-- that’s love, maybe, but it’s also codependency. All I’m saying is read The Summer Hikaru Died. /lh)