Okay everybody, I have awesome news
While it took me like forever to get all the bits together (ordered the wrong adapter for my programmer, didn't notice, ordered the flash chips twice, work, stuff) I can now report that Version 4.11 will boot with certain limitations!
I want to thank you all for the input, I wouldn't have guessed that an update could actually solve this!
Okay so what limitations then?
Well, at first the Turbo 040 is seemingly incompatible with bbrauns ROMDisk. This is very unfortunate and maybe I can help troubleshoot that (because the plain IIsi/IIfx ROMs work just fine with the Turbo). So this is inconvenient, but can be worked around (i.e. using the original ROMs).
But there is a second thing that is really annoying: The Turbo would completely fail to boot if I install any more than 8MB RAM.
This is happening with each and every ROM I tried (SE/30 Stock, IIsi, IIfx) - while the machine would boot up fine without the Turbo, installing it would always lead to the image seen in my starting post. What's interesting here is that the Turbo seems to somehow do it's own RAM verification - because if I run a stock ROM with 8MB of RAM, I can see this weird pattern for ~9s before it disappears (that's what other also describe when running the SE/30 with an IIsi/IIfx ROM).
Now when I install the Turbo, instead of speeding this check up (which is what I had guessed), it would now take ~18s for the same amount of RAM, i.e. it seems to be checking the RAM twice for whatever reason?
Does anyone have any idea how to troubleshoot that?
I already tried many different memory configurations (according to the specs, in reverse, only 1 bank) but as soon as I hit 8mb, it wouldn't work with the Turbo.
Thanks in advance again for your valuable feedback!