Considering you cry about invasion of privacy for 'choosing to continue watching' when the chapter itself had him acknowledging how serious just doing the act itself was (and was taken aback by how little Myne cared) didn't have high expectation to begin with.
He only decided to do this after seeing just how vast Myne's mana reserves are. However, in his eyes Myne is still a child, even if quite mature in some ways, but also immature in other ways. All in all, Myne has pretty much shown everything there is to see in her daily life. She's not exactly shy about her desires or projects. So, yeah, I wouldn't say Ferdinand had high expectations of finding anything troublesome, but by the looks of it he needed to see it because due to Myne's power, it's possible that ten years later she could destroy the city if she had some (figurative) demons hiding within her.
No matter how remote, the possibility would exist that years later Myne would be ready to annihilate anything that opposed her, if that had been a commonplace practice in her otherworldly background. Instead he saw a very peaceful world and still the same obsession with books. Then taking a bath with no sense of maidenly shame. It's 100% the same he would expect from the Myne he sees everyday.
What I'm saying is that this is technically enough. Any more, and it's not about Myne anymore, it's about him. Like has been said in this thread already, Myne most likely would want to show more since she ought to be enjoying this, but the dude needs to decide if it's something he should do, considering he's not interrogating an enemy but a rather a partner, and a child, to boot.