Yeah, thank you! You expressed it better than myself. I started to read because I fell in love with Shun and Mio, but they're barely there, there's no development between them at all
I would be okay with the story straying from them if I felt like their relationship had been mostly resolved, but it hasn't at all. If anything, Shun seems mildly annoyed by Mio's presence. I understand that is a dynamic you see in relationships at times (especially fictional ones), but they're usually coupled with sweet, sincere moments so both parties know the banter is just light-hearted teasing. You don't really get that here, unless it's internal monologues. I don't know, as Shun and Mio are now, they seem like they're each other's emotional crutches, instead of actually in love. So...not really in a place for their story to be put on hold for some angsty teenage pining haha.
I think that there at that point in their relationship where since they have been together for so long the passion they once felt is not burning at bright as it used to, they still love each other but maybe they are bored with how passive things are right now and just need something that can help spark their relationship
It it seems they're in this stage plus Shun is depressed and you can see that it has been slowly eating him alive, but I agree with OP that they should be the focus; if their relationship is staged I want to see them trying to solve it! But all we know after the time skip is that they're still together and then the focus has been on Fumi that, no offense, I'm not interested in, he's a great secondary character and 1 chapter focused on him should have been enough, but we're still at in and I really don't care if he's gonna get over his crush for Sakurako or still pinning.
As much as I love this manga, I feel like it's been ongoing way too long for Shun and Mio to not have progressed further in their relationship. Moving in with Shun's family has made their communication worse, and they didn't have good communication skills to begin with. They both have a lot of baggage, and as much as I do think they truly love each other, they're really kind of abysmal at expressing it. Shun, especially. At this rate it seems like we're heading for the relationship slowly fizzing out until they part ways. And maybe that's the point, for them to fall apart, learn from their mistakes, and reconvene.
I don't know, I just think there is SO MUCH still left to resolve with these two that focusing on Fumi makes no sense. He's cute and all, but he's not why I read this manga.