Its honestly not about the revenge at this point, its genuinely how they make the mc a meek bitch. It's become so common that I gauruntee i can make a bingo card of how he is a doormat and have that shit filled in 4 more chapters. I'm furious at that level of poor writing thats taken over the manga scene entirely
You're mistaking indifference for meekness. It's not that he's a spineless doormat, it's that he just doesn't give a fuck. A) He already wanted to retire, so being forced to retire and getting blacklisted is a non-issue. And B) His reaction to Kaori leaving him was fairly realistic, if a bit apathetic. In real life, people don't craft horrific revenge plots and face slapping when they get cheated on and dumped. They get their heart broken, lament over it for a bit (varies by person), and move on with their lives with a sense of distrust towards future romantic partners for a while (varies by person).
Expecting him to beat the shit out of the guy and stomp the cheating whore's face in (or whatever you expect a "not spineless MC" to do) is actual fantasy. That shit just doesn't happen in reality, or at least it's not often, and the guy would be arrested for assault in 90% of countries if he did do something like that.
Anyway, MC has already proven that he's not spineless when he took down the guys who attacked him and Karen, and he'll showcase more of these "I'll take action where necessary" moments in the future.