There's just not much Felix can do right now. That he still denies it or doesn't realize Sophie's cheating goes a little with the plot of these types of stories. There is already the route where Félix is most distrustful of Zack's intentions and it ends in three chapters. The problem I see is that Felix has nothing to compete with for Sophie against Zack, neither physically, nor sexually, nor psychologically, he is more boring, less masculine, even Sophie already equals them intellectually in many aspects, enough to be able to reverse. the situation with Sophie even discovering or showing her that Zack uses her, at most it could get her to leave him but I don't think she would go back to him let alone, as someone proposed, a relationship with Any. If he at least had the physical and sexual conditions of a "normal" person as in other games (for example, A Promise Best Left Unkept), I could think of a different and less bad ending for Felix even without Shopie.
I clarify that I have not played update 0.26
