Ah, I see. I only saw you throw N2- and N1- grammar points around so I had nothing to relate to. (And of course, I mean, why would you mention the simple and straightforward passages? Totally relatable.) On the other hand, books have that as well 
I think it’s very hard to assess and compare difficulties.
In my defense, I’m a very slow reader in any language. I once took a test that measured reading speed in English, and the test also measured the comprehension rate. They then explained that the average comprehension rate is about 60%. You know, wow, if my comprehension rate were that low on average, I would rather not read at all 
I’m aiming for 100%. Which is of course lowered by the fact that I still misunderstand things (ofc more in Japanese, not so much in English although it still happens) but I would not be fond of lowering it further on purpose by reading faster and skipping things
Sounds plausible to me. I have a video of somebody reading ~25 pages of a book out loud in 50 minutes (i.e. ~12k chars per hour), now imagine they wouldn’t read out loud, that would probably put them at 20k per hour. (I must confess I tried to read along just to see what it’s like, but at 12k per hour I can’t even take in and recognize the kanji any more 
)
Yeah, if you study on your own and don’t have others to compare yourself to, then I guess that it can be pretty hard to notice (the existence and level of your) improvements… If you were interested you could e.g. take some JLPT sample tests and stick to the specified time frames - and although you won’t know your exact results (because the grading scheme depends on the submissions of the test-takers at that time) you can still get a good impression, I’d think.