EDIT 4 - Sadly, probably the last one: Hi everyone, first of all thanks to everyone who dedicated their time to try and get me a solution, I appreciate you! However, in the end, I had to contact Steam support after countless failed solutions. As I mentioned in a comment, I hadn't asked Steam support to begin with, because I feel like they're either using bot-answers or some people, that barely read your message only to send you a link to their Connection issues-page. Sadly, that's what happened this time too. I did several replies to them being really elaborating, but in the end, they closed my ticket without the willingness to assist me further.
Here's the last 2 comments from the support ticket:
https://imgur.com/a/8Tex0yB
So, I'm guessing this is the end of the line for me. Probably something I'll never be able to fix.
If anyone, for any reason, have a suggestion on how I can contact some tech-dev guy or w/e from Steam, I'd be happy to hear from you! Feel free to PM me.
For the amount of people out there, who contacted me about having a similar problem, I'm sorry that I wasn't able to reach a conclusion for us...
Peace out
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EDIT 1: BREAKTHROUGH: Tried downloading something from a friends account, and I did it @ 113 MB/s no throttles what so ever - took 3 minutes to download Path of Exile - so the problem is apparently on my account. Hella weird.
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EDIT 2: Holy shit all the feedback, thanks a lot fellas!

So, let me update you on where I'm at right now:
- I tried going on my friends steam account, and downloaded a full game @ 120 MB/s no trouble, went to my own Steam account after, and had the same problem as always.

Then, I went on my laptop, logged into my account, and tried on that (With it using the wi-fi, only maxing out at 30 MB/s) also, no trouble.

SO, my downloads doesn't work on my pc with my account.
My friends accounts downloads works on my PC.
My laptops downloads works fine on my account.

How does this make any sense? So, the logical answer would be that none of my disks apparently can handle 1000/1000 internet for longer amounts of time.
But then, why would my friends account work on the very same pc????

Just for good measures, this is the Nvme SSD I have steam on and stores my games on:
WD Blue sn550 nvme ssd - 1tb

I posted this as a comment in the comment section, if anyone can make any sense of this please reply on that comment, thanks!

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EDIT 3: Update from Steam - 12/20/2022
So, as I feared, Steam shook me off with a "please try all steps on this FAQ Slow connection page [https://help.steampowered.com/da/faqs/view/5AC5-8056-E88F-F3FF]" which I obviously haven't tried at ALL after 5 years :\^) Lol, ofc I have, several times. Stating that I've had this problem for years and tried everything I could find on the internet including their help section didn't seem to make Ella from Steam support less confident that doing it again might fix it.
On top of that she had the nerve to end the message with this: https://imgur.com/a/IyprtUi (Telling me that I can't borrow my friends account to troubleshoot my own xd - oh well, rules are rules)

In response I wrote a long-ass-answer, in my best tone (I'm doing customer service myself) and did a full on explanation on everything that works and doesn't, literally citing this whole post to her, and now, all I can do is wait for a response to that. Just wanted to let you all know - thanks for being so eager to help and find a solution!
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I'm literally about to lose my shit.
I KNOW this is a common problem, but please, hear me out - cause there is no way this is normal.
So, through the last 4-5 years or so the downloads on my Steam client has been so damn slow even though I have good disks and good internet. I've tried SEVERAL download regions, firewall exceptions, fresh installs, you name it. Everything.

THE PROBLEM:
So, whenever I start a download on Steam, no matter the disk I do it on, it starts high (Goes to 100 MBps) it stays there for a couple of minutes, and then it just death-drops to 0. Stays on 0 for a few minutes, then starts going up again, but it only stays up for some seconds then goes straight to 0 again. It proceeds like this untill the download is done.
The only way for me to finish a download of 30 GB in a couple of hours (Which should only take MINUTES with my internet) is to pause and start the download constantly.
Picture of the download im doing atm: https://imgur.com/a/9rtuPa1

It's only happening on Steam!

THE TROUBLESHOOTING:
It's been some years, and through those years I've literally squashed the possibilities of it being either my internet or my PC:
- In that span of time, I've lived 2 different places, with 2 different internet providers and 2 different upload/download speeds - so that can't be the issue - and for that matter, I've also had different ethernet cables (Yup, I'm that far out, that I'm questioning the cables)
My current internet is 1000/1000 Mbit/s running through a Cat7 cable.
- I've gotten a completely new rig (Besides graphics) - Win 10, 13600k CPU, new mobo, new ram and with 3 different disks (1 SATA-SSD, 1 nVme SSD, 1 HDD) - Steam itself is on my SATA-SSD.
And obviously with that new rig came a fresh Windows 10-install aswell.
- As I mentioned I've tried countless download regions, firewall exceptions, I've sucked the internet dry of potential answers, and yet here I am, several years later.

I'm starting to think this is a problem with my account, and not Steam. If anyone's got any magic solution or any suggestions at all, I'll be more than happy to take it.

Sincerely,
Your boi