that spin off with a happy ending vibe,
but this one still has the revenge vibe,
wonder how it gets from one to the other
Nah, I think the side stories are meant to be a red hearing, with the twist being that it's all a plot.
In Chapter 3 of "Former Delinquent Girlfriend and Mister Earnest" you can see this exact scene, and it's portrayed as a cute memory of the couple, but here you can see what actually happened.
Most chapters of My Hero establish that MC excels at hiding his real thoughts behind a big, fake grin. Which, we can assume, he is doing through the seemingly happy chapters of "Former Delinquent Girlfriend". And also that he is totally broken inside.
He is so unhinged, that I wouldn't be surprised if he was planning to kill himself, after building a happy home with her, just to give her the same feeling of losing someone very important.