What you're describing definitely sounds like a fountain pen--specifically, an eyedropper (where the body acts as the ink reservoir), desk (long, gently sloping body coming to a point) pen--did it look something like this? https://cdn11.bigcommerce.com/s-5ko0zosub2/product_images/description/sheaffer/sh_deskpen_greymarble_2.jpg (Sorry, can't get images to embed)
For your thinish line requirements with a non-scratchy nib, I'd suggest a European/American fine or Japanese medium nib size (Japanese brands tend to run thinner). Most pens you can buy have some sort of cartridge, as well as a converter that would allow you to use a bottle--you can also get a cheap blunt tipped syringe or pipet and refill washed cartridges with whatever ink you want. I use a hot glue gun to reseal mine for transport, and peel it off before reinserting the cartridge.
I would recommend going to a stationary store/office supply store and trying some out! If you give us a general geographic area we can try to recommend stores near you that cary pens you could try--in Europe I've found this much more common than the US. If in-person testing isn't an option, I would tentatively suggest a pilot kakuno--they look like they cost ~11€ in Europe, and come with one cartridge of ink. They're Japanese so I would recommend a medium if you want to avoid scratchy at all costs, or a fine if you have some tolerance for a bit of feedback and want a thinner line. Any of pilot's converters should work, looks like they cost ~6-12€ depending on refill mechanism and capacity.
A kakuno or other budget pen won't look as amazing, but in my experience write almost as well as pens costing 10x more, and honestly if it's your first one you probably won't be able to feel the nuance yet.
This premise gets thrown around a lot but I actually disagree. "Every time people turn out" is always also thrown in there like some arbitrary thing--when I think the past several election cycles have shown that when there are younger, more progress candidates who make it past the primaries turnout shoots up. Courting the 3% uninformed flip-floppers by moving right is a losing strategy when you could be motivating your own party to turn out by moving left and driving turnout up. There's no money in that though, so dumb centrists get wooed