Did it and thanks to this guide the Index is working again.

For everyone searching the web: My symptoms were vertical white lines in my left eye. Got the new eye tube from iFixit and everything is working now!

The Guide needs two updates though.

Step 39 needs to show the black pieces and springs for reassembly as this was very hard. (I think one of the parts is missing in thge pictures of the guide ;-) )

And There seems do be a newer version of the headset that has a plastic plate over the bottom parts of the tubes that prevent them from being extracted. This part is missing in this guide.

BIG THX @samomio for the guide and iFixit for having the part!

Thank you for this guide! Helped me very much. And now my iMac is clean again :)