I think it was more of a mental conflict. I think Shuji has the tendency to be kind, but with his trauma, he's forced himself not to be other thing (even if he might have slight tendency for it too).
In vol.1, it was fine because they weren't serious but then at the end of vol.1, they're becoming more serious for each other and it reminded Shuji of his past failed relationship and his trauma.
I think when he was trying to get Itsuki into the subspace the first time, he forced himself to be more rough than he should/wanted (with the influence of his trauma too), but then he punished himself for it.
You can see when they were confronting the brother that Shuji did have the "forceful" dom side, but he's suppressing it. After he accepted both side, could he actually be a dom without the pressure. But since his tendency is a kind dom, we don't see him being violent. Just that he wasn't worried IF he was violent or not.
There was conflict and confusion regarding Shuji actions in vol 1 and 2. I don't know if it was the translations or the author but even now I don't get it.
Seems like his preference all throughout vol 1 is to be kind and spoil his sub. Which makes him a perfect partner for Itsuki because the author did a great job of showing how safe Itsuki felt being treated like that.
However, in vol 2 Shuji says he was acting he was kind because he was scared but he realized he wanted to be mean to Itsuki and Itsuki didn't really mind because he tought he could handle it. The fact that Itsuki doesn't react negatively towards the mean attitude of Shuji directly contradicts vol 1 and let's us believe he is indeed not satisfied which is why he didn't achieve subspace. This is fine since both of them will change togethere to be even better evenly matched as a pair but instead the writing leads us to believe that they want to change who they fundamentally are to be together even if they are not suitable for one another. Which in the end it doesn't matter because it is all scrapped and they keep their original inclination and turn around and have Itsuki achieve subspace for no apparent reason.