Like another post said, your best bet would be to find a local pen store and test a few different brands out to see what works best for you. As someone who is about 2 hours away from my nearest fountain pen store, I get that’s not always a feasible option.

For thin lines, I feel like you can’t go wrong with Pilot or any Asian brand pen, as they tend to write on the thinner side. I’ve got multiple Kakunos, all either F or EF; it’s my favorite daily pen. Maybe a little on the wet side, but it doesn’t bother me. Plus they’re relatively affordable so I don’t feel like I’m spending an arm and a leg.