One of the things I really like abt shumika as a ship is that their dynamic changes a lot depending on where they're at in their story, because one of the biggest Valkyrie themes overall is growth and change.

I'll start with when they briefly met as kids. Mika idolized Shu, even though he was the one to guide Shu home when Shu was lost, because Shu treated him with a kindness that Mika had never experienced before. This was so important to Mika that it changed his life, even though Shu doesn't remember it at all. It's what leads Mika to going to Yumenosaki and being an idol in the first place, and it shows this sort of admiration and idolatry from him. Shu is his God in a sense, and he is the lowly worshipper just grateful for his attention.

While they're in ex-valkyrie, Shu's attention is mostly on Mika, but he does acknowledge that Mika is "pretty" but doesn't go much farther than that. He's frustrated with Mika for not being perfect, and Mika internalizes this. He wants to be Shu's doll so badly. Whatever Nazuna has, he wants. But he's not Nazuna, he's not "small and cute and pretty" like him, and he doesn't think he's Shu's type. To make matters worse, he's brought on as a backup dancer, not because of his singing voice. How can he compare to Nazuna's angelic singing voice that Shu prefers? Mika here is mostly staying in his lane, he wants to become a great performer for Shu.

My thoughts lean towards Shu not really having feelings for Mika other than finding him attractive at this point, because his focus is mainly on Nazuna, and then on the stress dealing with the new live show system, up until the inevitable fall of Valkyrie. Because what I think really wins Shu over, is when he breaks. When he shuts himself in his home, becomes "useless" and falls from his throne. Nazuna leaves, he's done with everything. But Mika, the doll he didn't even prioritize, stays with him and treats him with kindness, even when he's uncertain if Shu will ever get better. Mika saw Shu at his lowest, and didn't falter. He has hope, hope that Nazuna wasn't willing to pause his life for. I do think Nazuna is fundamentally important in understanding shumika's dynamic because there are a lot of ways that I think Shu learned from the mistakes he made with Nazuna and took into account when interacting with Mika.

In ! era, Shu is harsh to Mika but in more of a "tough love" sense where he doesn't want Mika to be too dependent on him and sees that as worrying. I also sense a hint of restraint from Shu about how to treat Mika, because as we know he used to sing praise about Nazuna constantly and it ended up making Nazuna feel like if he changed at all, Shu wouldn't love him anymore, and that caused a rift even before the fall of ex Valkyrie. Praise needs to feel earned, if he tells Mika that he's beautiful and perfect and angelic, will Mika truly believe it? Will he strive to be better? Will he ever grow from someone who just lets Shu control his life and do whatever he says? I think while Shu does appreciate Mika in ! era, he doesn't entirely realize just how important Mika is to him until later when he realizes he's going to go abroad and be apart from Mika, which is why he tries to amend the harshness of their relationship in later ! era stories. Unfortunately for Mika... He gets anxious about that. Mika doesn't like change, he's scared of it. The consistency he gets from Shu is comforting, up until Shu starts being nicer, and then he's afraid that Shu is done with him and he's going to throw him out. He can't trust Shu's positive emotion, only his negative emotion. He wants Shu to continue to control him, because that's how he sees his worth. Shu wants him to find worth as a human, though, and Mika can't do that unless he makes the change himself.

In !! era things complicate again. Shu's overseas, and Mika is living on his own. When Shu first comes back, he tries to take control of Mika's life in a few different ways. He hasn't let go of his obsessive controlling tendancies, going so far as to cancel Mika's lease, throw out his "junk", and move him into the dorms. Unlike before, Mika does stand up to this. He gets visibly pissed off, and I think this is the big dynamic change between ! and !!, is that Mika is now allowed to stand up for himself. He has a voice, and doesn't blindly follow Shu. He still has loyalty and admiration, but he trusts his own feelings more a gives them a voice. It's the first step to becoming more equal. Though not everything is solved... In fact, it makes Shu a bit sneakier. There's a few lines where he mentions that he tries to hide from Mika whenever he's not doing alright, making sure he's dressed his best for their video calls so Mika doesn't "worry" about him, to the point where Kuro and ChiKI are requested by Anzu to go visit Shu and check up on him. Mika... doesn't know this is wrong, because Shu really doesn't want him to worry. He want's Mika to enjoy his life to the fullest, even though Shu isn't there. Despite this, Mika does miss Shu deeply, telling Sora in Shinsekai that sometimes he misses Shu so much he could cry. There's this new element of their relationship where they hide things from each other. This is also seen in Antique Legend, where Shu tries to give Mika an anonymous donation which ends up causing problems instead of just telling Mika directly. The culmination of both these points is seen in Raison, where Shu implicitly decides that Mika is going to move in with him after graduation and buys a bunch of furniture for his arrival. This isn't a conversation they have, it's based off assumptions that Shu makes. Mika would jump at the chance to live with Shu in a heartbeat, but he hasn't been invited to, so he thinks that's off the table. Shu can't ask Mika, because he doesn't trust that Mika wouldn't just say yes because it's a demand from Shu. The trust issues are really pertinent in !! era. Shu makes tons of assumptions about Mika without clear conversation, and Mika doesn't see his importance to Shu. In Hermitage, Shu's big speech about how much Mika means to him is said while Mika is hypnotized, and I think this is a greater allegory towards how Mika views the relationship himself. He can't imagine Shu loving him because his self worth is low, almost as if he's hypnotized in that he's blind to all the affection that Shu does show him because he doesn't think he deserves it. Shu straight up tells him how much Mika means to him, but Mika... can't hear it. He can't internalize it. Though at the end of Hermitage some progress is made, Mika awakening to say that he's starting to remember bits and pieces of what happened. Hopefully he can reflect on their three years together, and realize that Shu does love him, and he is worthy of that love, he's not just saying things to make Mika feel better.

So as for preferred dynamics
1. Ex Valkyrie - Mika is crushing hard on Shu in a very idolizing way. Shu's his everything. I think Shu feels a smidge conflicted/guilty about finding Mika attractive here, so he makes up for it by insulting him. (Shumika but mostly through the lens of Shunazumika)
2. Nursing Era - We don't know too much about this other than that Mika stayed and took care of Shu. Mika loves "fixing" broken things, and we know that this means a lot to Shu from what he says in Hermitage about how Mika wouldn't ever throw him away. Shu has temporarily relinquished control, while Mika doesn't puppet him, he does take care of him. (Mikashu)
3. ! Era - Shu wants Mika to become more human. He's risen again from his breakdown, and he's ready to be a performer again. He's back to the idol that Mika idolizes but we do see some of his faults. Mika's insecurity about the state of their relationship comes up a lot in later stories. (Shumika)
4. !! Era - Trust issues + long distance relationship. They only see each other in person sparingly, and especially Shu pretends things are going better than they are when he video calls. Their codependence is a little less saturated as in ! era, but new issues have cropped up. (Like 75% Shumika, 25% Mikashu)
5. !!! Era - What I predict for this is Shu becoming a lot clingier on Mika. We know he wants Mika to move in with him. We know Mika is his "only patner in the world" (despite calling Nazuna his soulmate years ago but that's for another essay). Mika is finding his ground as an artist, I'd like to see him take on a more dominant role, in that he can push his creative decisions further, and even disagree with Shu without fear. Mika is blossoming into becoming an equal with him, and I think this would be something to explore. (unknown yet! But I'd like to see 50/50 Shumika/Mikashu :3)