personally i don't really ult any group precisely because of this reason: i struggle to define when i "ult" someone as opposed to when i stan them. for me stanning is (a) listening to and (on the whole part) liking the entire discography at least once (boa's a teenie meenie bit of an exception but shush not my fault she's got a vast discography befitting someone who's been an active idol for 20+ years!) and (b) i keep up with (some) of their content. like, as much as i love f(x)'s discography, i don't keep up w any of the members, so they're firmly in the "casual" list. so some groups are my main focus at some times, while others are not. it's fluid, hence i can't decide when i "ult" someone or not.
i do have ult biases, though that's kinda fluid too, cause like if i'm paying more attention to a group at a given point in time, i'm also paying more attention to my bias in said group. like look at me. i have a taerae layout. he isn't an ult bias or anything, it's just that i'm paying a lot more attention to zb1 these days, so i'm paying more attention to my bias within zb1... who happens to be him.