My new favourite pen: The Uni Kuru Toga 0.5
tl;dr: After years of tinkering with fountain pens, inks, nibs and gel-pens I found the possible solution to my "pen-issue" in the complete opposite: in a mechanical pencil

I can with absolute confidence say that, over the past six odd years, I have amassed, erm.. collected, tried and used quite a few fountain pens, inks, nibs and subsequently fiddled with ink/pen/nib combinations in the search to find just the "right" setup. Throw in paper and notebooks to go well with said pen/nib/ink-combo and you are dealing with quite a bit of variables.
Suffice it to say the whole procedure had been quite the mess and it not got any easier as many of those "components" are not available locally, thus has to be ordered.
Maybe I am too picky, I don't know, but I prefer my pen to write consistent in color and width from beginning to the end, over the whole page and overall
The "problem"
The problem I frequently had is that in my experience fountain pens the to write darker in the beginning and then gradually lighten or change to the real "color" of the ink over the course of a writing session. This takes usually a few lines but then at the least the color stays consistent.
Until I take a break... or continue to write later, when the whole thing starts again.
I think this is fine when writing a longer text, but when adding entries to, say the Bullet Journal over the course of the day, the different shades of color are somehow messing up the page, giving the page a somewhat smudgy look and I don't like that.
At many points in time I had been so annoyed at the inconsistency of fountain pens that I switched to gel-pens. There the situation though is not that much different.
Gel-pens tend to be more consistent, though tend to get gradually lighter to the end of their life. That seems to happen often so slowly that it is not really obvious in the first place, but get's doubly so, when then pen gets a refill and are presented with two shades of color on the page.
I know, these are stupid things to worry about, but they do annoy me and my preference of more muted colors, blacks such as the Platinum black, which is almost a dark gray, and on the gel-pen side, Uni Signo DX, which I would consider also more in the "off-black" spectrum, seem to only further enhance the issue.
In comes the Uni Kuru Toga mechanical pencil.
The Uni Kuru Toga mechanical pencil
I have of course used mechanical pencils in the past, but don't think that I have used them for longer writing since my school days. The pencils I used in the past, natually also have the issue that the shape of the tip is changing as well when writing.
But you know that of course.
The Uni Kuru Toga however has some sort of mechanism that the turns the graphite every time one stops writing or lifts the pen, thus ensures a consistent writing experience. When I learned about the pen I ordered one just to give it a try. Maybe it works.
And it works like a charm and I had been hooked ever since and use it all the time.

Writing angle and applied pressure aside, the color of the line is consistent from beginning to the end, over the whole page, after breaks, in the shower... every where. Consistency all the time.
Even the choice of paper doesn't matter "that" much. It naturally does affect the line and color, but still.

Currently I am using 0.5 "Roulette" model (at the least so I believe - the one with the indicator where one can see the rotation) with a 2B Pentel AinStein refill and consider this to be very nice experience to write with.
It's just great! Now I only have make sure that I don't think too much about how much time I wasted spent figuring out the right setup.