Also, while it's far besides the point, I think Lunaxia's got a wrong read on Hythlodaeus, who I think is just realistic about his abilities and limitations rather than 'worryingly hard on himself'. He's not really down on himself in any way except his deficiencies in magic. He's quite happy, quite friendly, knows his stuff and is happy to share it and use it when he has to. By all appearances, he's quite good at his job and quite comfortable with that (except for the part where Byregot's doing most of it instead). The problem is that when the chips go down, suddenly the situation is calling for those skills that he's confirmably not good at. So I don't think Hyth is an unfair judge of his own skillset at all, especially given Emet and Venat don't exactly call him wrong about it--and you'd expect Emet espeically to talk up his skills if Hyth was just down on himself for nothing.

Now Erich, the other established weaker-than-average Ancient, definitely is hard on himself, but that's an established flaw that he overcomes as part of his journey. Which itself says to me that Hyth doesn't have that flaw; we know how the writers write 'an Ancient that's hard on themselves for lack of magical ability', and not only does Hyth not act like Erich, but Emet and Venat don't act like Themis.