It's Hinata, not Ushijima that has a massive ego

Well yeah, but I never mentioned Hinata. I don't think anybody reasonable would claim that Ushijima had a bigger ego than Ushijima, but that doesn't mean that Ushijima doesn't have one.

it's precisely this that Ushijima doesn't like about Hinata.

Specifically, he doesn't like that Hinata has a massive ego despite not being very skilled. He thinks Hinata isn't good enough to warrant that kind of confidence. Conversely, he believes that he himself is, which is a definite element of having ego himself.

He also strongly criticizes Oikawa for not choosing to go to Shiratorizawa, because he doesn't understand that Oikawa really dislikes him for being gifted and wants to prove through his setting that he can bring an arguably average team to beat an elite one that relies on genius individual players.

I'm not sure how this is helping to prove your point. Ushijima not understanding Oikawa isn't an argument for or against him having an ego. Hell, the way he frames it makes it come off as though he believes attending Shiratorizawa is the only choice that makes sense, which displays a pretty intense sense of self-importance.

Ushijima isn't confident because he's got an inflated ego, he's confident because he's extremely objective and straight forward.

It's both. He's both straight forward and has an inflated ego. Tbh, though, he's far from objective. You said yourself that he couldn't understand Oikawa's desire to beat him. Any objective person would be able to understand that. What you're framing as objectivity is, imo, a very self-centred worldview and a lack of empathy.

As for the stuff about Tendo saying that he saw a new side of him... Ushijima doesn't get super visibly emotional. Getting more carried away than usual isn't really related to this at all.