1. Some of the hundred or so I owned during my school years were thick as a pickle, but even the thinnest was around 3/8" diameter, and very uncomfortable. A Cross ballpoint is perfect for me; from there they become progressively more hand-cramping as the diameter increases.@shutterbug:
I have owned dozens of them...leaky, fiddly, smeary, too thick for comfort like trying to write with a banana.
Bear with me if you will... there are a number of issues here.
1. You have the wrong size pen. The finger length to penis size thing is not correct. Pick a pen that fits your hand. If you like slim, anything else will be horrible.
2. The pen itself. You haven't specified brands/models, but some are leakier than others.
And most importantly:
3. Fountain pens write at a 45 degree angle. If you try to write up-and-down with one, it will be a disaster.
2. I owned a few Waterhouses, a few Parkers, a Montblanc or two, and a ton of Schaeffers. One of the latter was 14k and had a pneumatic piston to load the ink, rather than a lever to compress a bladder. It was the least leaky, but all of them tended to leak inside the cap a bit and if I forgot to wipe it down, the ink was all over my fingers. That's mainly what I meant by leak. The only ones that ever actually dumped their ink supply in my pocket were those el-cheapo cartridge-loading Schaeffers.
3. I'm not about to make pen-writing "a hobby" with a set of inane rules. "You have to hold your pen at a 45 degree angle" is as idiotic to me as "you have to dry your tobacco". I don't hold a pen "up-and-down" but I have never held a protractor up to see what angle I hold it at. I'm going to hold it at whatever angle is comfortable, and it may change from time to time. I may even need to write against a wall, or upside down. I rule the writing instrument, it's not gonna rule me. Fountain pens were state of the art at one time, now there are much less troublesome and more convenient pens available. I happen to like the wet-ink rollerballs best. The ones with quick-drying ink. I am lefthanded, those smear the least.
I'll agree fountain pens are beautiful to look at. Other pens are soulless utilitarian object by comparison.