UPDATE: I managed to install stock firmware! (uff....)
I downloaded oreo stock and ran it through Odin, somehow it worked.
Also Tkkg1994 I tested the thing with OEM unlock again. It is true that when you turn developer options off, that turns OEM unlock off as well. Pretty... Inconvenient for the amount of trouble it gives. If you could just give a quick last update to the Superman 3.0 where it's impossible to turn OEM off, I think everyone will be happy and thankful for it.
Either way, gonna try and install 3.0 again and this time not ruin it all!