Jetstreams are great. Worth pointing out that you can buy refills in various form factors, including Parker style that'll fit the Rotring 600:
https://www.jetpens.com/Uni-SXR-600-Jetstream-Ballpoint-Pen-...
I also recommend the 800 mechanical pencil which has a very satisfying twist-retract tip.
Why? Why would you do this to me?
Sighs and busts out the credit card.
Edit: it’s on its way. Curse you.
The discovery of the jotter style refill led me down a path of trying many different inks for my Rotring 600. I ended up liking the OHTO ceramic rollerball quite well, partly for the ink, and also for the appearance of the nib, which I think suits the Rotring 600 nicely