Jonah is so hard to write because he conceals so much of himself. you can deduce a lot from what he doesn't say but to what extent is his behaviour an act? does he genuinely like cake or is it an act to fit the image of slightly incompetent, personally impersonal boss? he plays up the evil villain act when confronted but also he has such a tight grasp on his emotions. even in a stressful situation like with Leitner or Martin burning his old statements and personal letters (I do like how obvious was his struggle to collect himself, to not outwardly react, up until he started mind torturing Martin). but we know so little of him and his thoughts and struggles and what made him choose this
in addition he is very... inert (?) in his interactions with others. I have already talked about how he is metaphorically in the Panopticon but there is always a certain distance between Jonah and every other character. the closest person to him is, of course, Jon. and even he is kept at arm's length (he dismisses Jon in mag 92 and is not exactly responsive to Jon's show of vulnerability despite him opening up previously to his past a way to put them on the same level which is apart from others)
I get frustrated by this too- in a podcast in which he is the main villain, the person the titular archives is named after we really don't get a whole lot from him. We have the letters, his statement/ ritual, and a few offhand comments. The only real concrete thread (as far as anything is concrete in his character when he gets so little airtime) is his relationship with isolation- how he keeps everyone at an arms length to watch and eventually discard and admired Gertrude for it. Which in a later episode was revealed to be her being close to the Lonely. Stepping away from cannon into pre-cannon though... There are a lot of dots that can be connected regarding Robert Smirke's timeline (That weren't addressed in podcast but he's a real person so I argue they count)