If you like fine line nibs maybe you'd like a rapidograph or isograph pen. I use a .001. It's a fine or finer than a single hair and I love it. The Pen nibs run from .001 up to 1.5 and above. They are also refillable so no waste. The pens do require maintenance however. If you aren't going to use them for a week or more you must wash them out. Just hold under a stream of water to flush the ink out of the nib. If you forget and the nib gets clogged simply soak in ammonia water mix.
i think the notebooks are meant to be broken in! im new to writing down notes, but from what i can tell the notebook that you said was hard to lay flat can handle being broken in. i only need it for notes and i like how affordable it is, but also i think it makes sense based on how nice the products are. normal paperback books dont lay flat either... im breaking them in pretty hard and the stitching feels strong. okay, maybe not so hard, the glue wont stay. but it's not really usable without breaking them in :/ actually, i stick to thinking they should be broken in. it is only an issue with the cover page that is not stitched together with the inside pages. do not break in the cover page, that's it.
I never heard of muji before. I like those notebooks though. Maybe, if they have a website, I could order some and tell them I heard about them from you. Maybe them you could get some free stuff