The boss should've just disappeared after she rejected him to be honest, there was no need for her in the story after the main character had his heart broken.
It could've gone like this:
MC has heart broken by boss -> Sayu is introduced -> Sayu helps MC recuperate -> Kouhai comes in to stop MC from falling in love with a minor (unhealthy behavior) -> MC falls in love with Kouhai and both become Sayu's guardians/waits for Sayu to become legal, dates her, and eventually has a happy end with marriage
There's a lot of different ways this story could've gone without the boss being there, since the introduction of the Kouhai brought endless possibilities.
I like wish-fulfillment stories because I know they're not real, but the boss brings this story too close to home.
If the author is trying to go for a more "real" approach in the story, it's working and many of us don't like it. Kouhai could've taken her spot just as easily, and she has been trying to. Sadly, the author wants drama and this is what the boss is all about, drama. I'm still going to keep reading because this is also my type of dumpster fire.
If you ping me, I won't be responding since this is just my take on this story, thank you for reading.