Cimorene

TWSBI is a fairly sturdy brand and they are good about replacing parts, but their clear plastic (resin?) is prone to fatigue and breakage. I've broken one already but I still have two more because I prefer piston pens to converters and they are SO much cheaper than Pelikans, but you get what you pay for. However, the nib is also much stiffer than Pelikan's nib, which is less bouncy and fun to write with to an extent but it is also more useful for some use cases, being less fragile.

As far as writing thin and thick lines - the essence of italic-style calligraphy which are all my favorite styles... although I think you are maybe talking about freestyle handwriting and not specific calligraphy 'styles' where you learn the stroke orders and historical forms? However, either way:

A stub nib is designed for everyday handwriting as opposed to calligraphy and the corners of the nib are rounded as opposed to the pretty sharp corners you get on a 1.1 italic nib. The Pilot Plumix is supposed to be an inexpensive way to test these - I can't remember if that's the one I had or not, but I bought one that was specifically for calligraphy and it was a lot scratchier. The rounded edges on stub nibs in principle mean the shapes aren't as clean and crisp. But that also means that stroke order doesn't matter as much and you can write faster/more casually without paying attention to the angle the nib strikes the paper. The 1.1 stub that I kept is a Lamy nib that can go on my Safari. Of course this is converter-filled so it's not as elegant as a piston pen (but I'm not buying proprietary cartridges). The collective wisdom in the fandom seems to be that Lamy stubs are a bit flatter and more italic than some, but it isn't scratching the paper up. And of course if you have a Safari already, you can try it just for the price of the replacement nib.