Okay, after checking everything, reinstalling systems for 100th time, I finally found out what happened: it was the damned HDMI cable.
I used the raspberry branded cable ever since I got the first rp. Now, the confusing part was that I was able to boot the rp OS normally and I could get it on my screen etc. After hours of fiddling with almost anything and everything, I finally unpacked a brand new HDMI cable and...it worked. I could actually boot all my sd cards and the LibreELEC I had on them. Once booted, I can switch to the original cable and the screen would remain.
IT WOULD NOT BOOT WITH THIS CABLE
I am confused af but relieved as well. I'm off to buy new cables tomorrow.