yeah I really like when a story has side couples because they do serve a narrative purpose. Something that you'll notice is that side couples tend to develop at opposite speed to the main couple.
So in a story where the main couple gets together quickly the side couple takes more time, and in stories where the main couple takes a long time to figure themselves out the side couple will come in quick and give you something to look forward to in the middle while the main couple is still full of drama.
In much the same way in this story the main couple gets a lot of development early on, and even gets together pretty quickly so now that the main couple is mostly free of the drama that makes a story the side couple steps up to give the story something to do while also allowing for the main couple to be together without needing to end immediately.