I've literally read books with just stars and a moon on it. So I don't have a good take on what I'm attracted to.
However I do know what is a red flag for me. Having a character that is not the MC on the cover. This often shows as a pretty girl on the cover while the MC is just some dude. I'm not a harem nor smut fan. I've read them in my 20's, but it's all just cringe post 30. What's this got to do with a girl on the cover? Well harem and smut often show romance targets instead of the MC.
I never understood this, since really it leads to a higher drop off. People can flip the switch for the smallest thing, but straight up lying to them isn’t taken kindly.
However, it does work to get a massive amount into whatever the media is. I suppose it is a trade off in that sense.
Personally, I do not like misleading people. So I make things clear. Each book’s cover would have the MC.
Such as Books 1, 2, 3, 4.
5, 6, 7. These changes to another for each, as it covers a new perspective. With the lead taking quite a backseat.
Book 8 and 9, while these are a mix. These covers return to the lead of the series, as it points to them again.
Characters in Book 5, 6, 7 become close friends to the lead, so their stories are important in Book 8 and 9.
As well as especially important in four new added scenes in the series finale.
Point is, I like being straight up. I specifically label 5, 6, 7 as Yuri. So peeps know what is in them. And Book 6, has an additional warning for a lot of gore.