What confuses me about the OP is that almost all fountain pens come with a converter so you can use bottled ink. For the most part you would have to get a vintage pen for something that can’t take cartridges at all.
The small (38 mm) cartridges simply do not hold that much ink; the 73 mm ones are more reasonable. The standard converters look about as long as the long cartridges, but many of them only hold as much ink as the short cartridges. With a dedicated piston/vacuum mechanism you get at the very least as much volume as the big cartridges, possibly significantly more.
L2K ink capacity ; 1.4 mililiters
Most converters hold 0.6-0.9 Ml
L2K holds 1.4
its very durable, handles pocket duty well, the Makrolon ages like leather, gaining a patina with use, downsides? the cap ears might be under the fingers, the ink window isn’t exactly helping, and you really need to like your ink, 'cause you won’t be refilling too often 
the styling can be polarizing…
I also have a Waterman Phileas (M) (converter) and a Pilot Metro (F) (squeeze converter), my L2K is an EF, but L2K nibs generally write one size wider than Lamy steel nibs or Japanese steel nibs (pilot), the L2K nib is 14K gold plated with platinum or Rhodium, depending on generation, newer ones are Rhodium
Yes, 1.4 ml = 1 long-size cartridge, more or less
Get on these guys’ mailing list. Every month they send you a catalog full of pretties ranging from cheap to ridiculously expensive. I’m sure you can find something they offer that will fit your needs and budget.
There’s also the tactile response. It’s subjective, of course, but for me, writing with a fountain pen just feels better, moving across the page. And I feel like my handwriting is more legible and crisp when I’m using my Nemosine or Pilot. Placebo effect, of course, but no less real for that.
I agree, it’s all about the feel, a FP needs NO pressure to lay down ink, the nib simply floats across the paper, and you get a slight bit of feel from the paper, you’re not pressing a stainless steel ball down on the paper to put down a rubbery (ballpoint) or gelatin (roller ball) ink, you’re not pushing the point into the paper, the feel of a FP is effortless
you get this with even the stiffest steel nibs, and with the more flexible gold nib, you can get line width/heaviness variation, giving your writing personality
FP ink is liquid, and the ink itself has it’s own "personality, and interacts with the paper in it’s own unique way, differences in shading, tonal gradation , feathering, there are inks that are completely solvent resistant, permanent and actually bond with the paper fibers, shrugging off the strongest solvents, short of physically destroying the paper, you’re not getting the ink off, Noodlers refers to these properties as “bulletproof”
ballpoint ink sits on the surface of the paper and can be easily removed with simple alcohol, some rollerball pens have “bulletproof” ink, and that ink straddles the line between ballpoint ink (sits on the paper) and FP ink (bonds with the paper)
why not give FP a try, go to Amazon, buy an inexpensive Pilot Metro, or their disposable Varsity, and try it, it’ll be a new experience, and you’ll be fighting ignorance…
https://www.amazon.com/s?k=Pilot+Metro&ref=nb_sb_noss_2
be warned though, this can be an EXPENSIVE hobby!
Some fun inks;
Noodlers Baystate Blue an eye-melting Cobalt blue, very temperamental, doesn’t play nice with other inks, best to have a dedicated pen for it, or do a complete field strip and clean if you’ve had other ink in your pen, fussy, temperamental, but oh so unique, great ink for a dedicated pen (the Metro’s a great choice for BSB)
Noodlers “Polar” line of freezeproof inks yes, you can leave your FP in the car during subzero temperatures (but why would you) and the pen will still write!, conventional inks will freeze and damage the pen
Go to Fountain Pen Hospital, click on the “inks and refils” link, try to find that variety in ballpoint or rollerball…
Be warned that the Varsity can be a bit hit or miss. I’m not a fountain pen person by any means, but I thought I’d give it a try. I ordered the rainbow 7-pack Varsity, and the standard blue ink pen was dry on arrival, but they did replace the pack for me. I then ordered a dozen-pack Varsity blue. The first one ran dry within a couple of weeks. I’m on the third of a dozen now, and I’ve had them since November, 2018.
I don’t use them exclusively by any means, but they do provide a nice flowing feel. The most difficult thing for me is to remember not to press down when I’m using them. That causes problems.
Personally, I don’t think it is a placebo effect, but it does seem to contribute heavily to a mindfulness about one’s handwriting. There is a symbiosis between using a fountain pen and improving your handwriting – using a fountain pen makes you more aware of your handwriting, and a desire to improve handwriting can lead to using a fountain pen more often.
Using a fountain pen is generally not as convenient as using a ballpoint/gel type pen, but there are advantages, many of which have already been mentioned, such as a better feel on the paper, and greater choices in things like nib sizes and grinds, and ink colors. There are some disadvantages, such as the need for better paper rather than cheap copier paper, the need to re-ink more often (at least for some fountain pens), the tendency to dry out and fail to write if left unused for several weeks, the possibility of leaks and spills, generally higher costs, the need to clean out pens from time to time. So it’s a trade-off that you pretty much have to want to make. I only made it after I retired. I perfectly understand anyone who isn’t interested, but I welcome any newly-interested.
the only pen that ever failed on me was a clear Waterman Kultur, that I accidentally left in my car during a freeze, the feed was damaged when the ink expanded
every other pen I had was fine, currently I have a Pilot Metro that my nephew gave me for my birthday last year (i introduced him to FP, so it was his way of saying thanks), I gave my med. Waterman Phileas to Mom, and my current daily is my L2K with Noodlers bulletproof black, 100% reliable
another advantage of FP, they work 100% on that insipid thermal receipt paper at cash registers, the thermal coating on the paper clogs ballpoints and rollerballs are temperamental, FP ink works fine, just needs a few moments to dry
as far as re-inking, the L2K has such a prodigious capacity that I occasionally get bored with basic black and purge the pen into a mix bottle, then re ink with something different (a mix of Noodlers Black and Noodlers Polar Green, ATM)
oh yeah, another advantage, you can mix inks and make your own unique colors/tints…
Like any liquid ink, some fountain pen ink is waterproof and/or “bulletproof”, most aren’t; some work great on even the cheapest of paper while others will bleed/feather like crazy; some dry instantly on the page while others require time or blotting. You really have to experiment to be sure. Just make sure the bottle says it is ink for fountain pens so you don’t trash your nice pen by filling it with iron-gall ink, Chinese ink, technical-pen ink or the like.
I particularly love pens from Autumn Summer Co. They’re a small business based in Maryland so if you’re American and prefer to support local businesses, it ticks those boxes too. They also do international delivery if you’re not. $50 will get you a beautiful hand-crafted fountain pen in a choice of 4 different woods (they also have alternatives like a bolt-action style pen and a pen made from whiskey barrels). They also do custom engraving if you want. It comes with a converter so you can use it with a bottle of ink (it’s the kind of converter that has a little plastic knob on the end that you twist to move the ink up or down through the converter to fill it or empty it - sounds like the kind you are referring to?). Haven’t had any leakage issues, writes wonderfully and is very well made. It’s more of a fine nib but not super-fine so it might suit?
wow i havent seen a pen thread in ages… i remember some of the old ones …
Worth keeping in mind that fountain pens were too good for Marcel Proust 
For him, nothing but cheap steel nibs and wooden holders and school composition notebooks bought in bulk.