So here is my conclusion.
At first: IT WORKS!!! I am now fully operational in DSM 7.01.
Here is my Story: at 18oclock i created a boot stick with Win32Diskimager and the image from -> tinycore-redpill
I followed the instructions given by the readme and all seems fine -> My Synology booted with the new stick, told me that it seems i changed my harddrives and opted me to migrate data -> so far so good... DSM installation started until about 60% jumped to 100% and then it told me the system reboots. The 10 minute countdown started.
after the reboot, i was stuck at the migration screen and tried this a second tim with the same outcome.
So to check if i done everything right, i took an empty harddrive plugged it in and disconneted all other drives... tried installation on this factory new disk... same outcome...
every boot brought me to the installation screen of dsm... couldnt get it to run.
The problem now was... on the first attempt with my whole disks dsm installed and raised the Version from 6.2.3-25426 Update 3 to 7.01. so i couldnt boot with my old juns loader anymore. <- This is the point where i thought ... i need to setup my whole system new...
3-4 hours later after trying and erroring with several USB sticks, and installalling Apollolake, Broadwell, Bromolow Versions, i found the solution that could have saved me much time:
The serial generator provided with the tynicore-redpill command -> "./rploader.sh serialgen DSXXXXX" gave me nice looking serial and mac combinations, but they dont work with 7.0.1-42218. Seems like the dsm installation process checks them so it fails migration/fresh installation.
Finally i took the real Serial Number and MAC adress of a real DS918+ and put it in the "user_config.json"
The installation and migration now worked without problems!!
side note: at first i took the serial from an old DS215j of my parents which run fine with juns loader <- no chance, it seems you need a matching serial of the same type of Diskstation you wanna install (DS918+ of my coworker in my case)!
If you try upgrading i highly recommed -> DO NOT try it with false serial mac combinations from the tinycore generator.
Even if this worked for some of you with older dsm versions prior 7.0.1-42218. <- Spoiler: for me it didnt even work with 6.2.4!
So now almost 90% of all settings are migrated automaticaly (dockers, shares, users etc). No data is lost. I am happy! <- this was arround 01:30 in the morning
After all i think, if i had done it right from the start with a real SN, the whole process (incl bootstick generation) could have been done in about 30 minutes without problems.