I found my solution! For me it was the programmer. I used the Xgecu T48 / TL866II -Programmer that is usually used for EPROMs. Although it showed that the programming worked and could even read back the written data, the Controller just didn't work. I then used an Arduino as a programmer and then the card just worked.

WOW! Great job @Physikant!! 🍷

In the meantime, I tested this CMS upgrade project https://www.youtube.com/watch?v=iGlIgqNpK50 ; and worked perfectly on my "QuickShot: Sound Machine QS803"sound card (SB2.0 licensed clone).
For anyone interested; links for the already compiled Rom file, and for the (not fakes)microchips sellers are in the video description.

Interesting, I also used a TL866 programmer ...

So I guess that was the issue all along !
Thanks for the info !

Actually maybe it needs some specific flag to be set for it to work 🤔

Im usually not a fan of arduino and the whole crappy-but-simple-mentality, but since I probably will never program this controller family again, I went the most simple route I found, which is this tutorial:
https://www.instructables.com/Program-8051-With-Arduino/
I didnt use a breadboard for 10 years now, but it was really quick and worked flawlessly.
So I can recommend. I used AVRdude.

Well ...
I wanted to check if I missed something in the TL866 that could flash the chip properly (flags, etc).

Turns out that when I read back the chip for good measure ... it was blank 😓

I don't know how this happened, I'm pretty sure I flashed the damn thing. Maybe the TL866 firmware back then was buggy and didn't actually flash those chips properly ? Maybe bad connection that somehow made the programmer "think" the chip was properly programmed ? anyways, I flashed the chip with the firmware ... and now it works.

Thanks everyone who tried to help me !