I still wish they got punished somehow
After some time sitting with this chapter, I'd actually argue they were.
Ryouta told his mom to her face that he at one point resented her, but also that her actions lead to him discovering a family and friendships that are super important to him now. He didn't forgive her, merely acknowledged that he also couldn't let his resentment get in the way of what he had now, which is why he sought closure in this moment.
And, him being this kind in his delivery of his farewell is an actually-effective example of 'taking the high road'. He demonstrated that his dad still managed to raise him into a respectable and honorable person without her help; to the extent he could set aside his grudges and trauma to save the life of a person he'd only recently met.
And he still walked away, choosing his Found Family over her. And every time they look at Suzuka, they'll be reminded of the son whom they let get away. Ryouta, for his part, didn't have to resort to pettiness or ugliness or vindictiveness or ill-will, and that means they can't even smear his character or malign his treatment of them. He's literally better than them, and proved it, and then moved forward in his life with people who actually cherished him, and he let them both know that.
It's not vengeance, but it is a statement of finality delivered without malice or ill-will that they will have to carry in themselves for the rest of their lives. He didn't have to condemn or punish them; they'll do it to themselves.