For the sake of this thread an update.
The guide by gr5 worked. I had to repartition and reflash, after that the recovery SD worked.
UM3 Partition sizes were exactly the same as in the guide. I used the eMMC version (the zip file, not the 7z) which worked and booted fine (maybe I have the eMMC version or the eMMC version work also for NAND? No clue, it worked).
The front board made an annoying beeping sound while running jessie, so I disconnected that. Watch a tutorial on how to replace the display and unplug the same cable to get rid of that while repartitioning (it gets really annoying)
Other then that it worked amazing and was a really doable procedure.
Many thanks to gr5 and everyone here on the forum, you are amazing.