Normally this is the kind of manga I like. No idea why I didn‘t enjoy it ...

I think because its a bit of a bumblefuck... Could be the translation and it could be because its kind of creepy to be messing with your Ex's much younger brother.

I feel like the storytelling is just rushed. The characters have no time to develop their feelings properly. It was the same in "MODS" where the seme wasn't even gay - and then suddenly he is so much in love and it gets sappy and cheesy. And the "brother of your ex" thing in this story is not helping either.