I've had an absolutely unexpected streak of luck
So originally I've been searching for a very likely lost and deleted YouTube video I watched like 15 years ago about which I forgot just about everything besides very vague details and I've been searching my computer if there's maybe some old version of a list of songs I wanted and liked besides the current one (though at that time I wrote in a paper notebook and the one I found has no more music-lists in it).
During that, I came across a folder that apparently contains my Ebooks I collected several years ago.
I thought I had lost them after moving the file (and quite a bunch of stuff I rarely used) exclusively to an external hard disk drive because I desperately needed disk space on my PC and I had nowhere else to store them since at that time I wasn't able to access the private cloud storage my dad had set up for us.
Said hard disk drive malfunctioned in a way that I couldn't access the data on it, despite the computer recognizing it when I plugged it into the computer. Even my former favourite colleague who used to take care of our IT at work and his friend who owns a small IT business and is extremely thorough with these kinds of stuff were unable to get my data back.
I don't really remember which stuff I put on that drive exclusively since it was data I didn't use on the regular (no pictures, definitely) besides my Ebooks and the stuff from applications - but now I have found at least some of those.
And the music a friend shared with me, but she'll bring her hard drive disk again when she visits in November so not even that is lost.
So moral of the story:
1) Back your stuff up in more than one place. The 3-2-1 rule is recommended - 3 copies, 2 different mediums, 1 of those mediums not at your home.
(If you need a lot of free cloud space - Terabox offers 1 terabyte for free. Mega offers 10 GB. Heck, when in doubt create different accounts - even using different services - and split your stuff up if you don't want to pay money for storage space.)
2) For the sake of your data, don't pull external hard disk drives or USB sticks out of your device without ejecting them first. Because apparently that was the cause of my problem.