UPDATE1
First part of the script went fine, but after upgrading to 7.2 suddenly my NAS detected disks 1 and 2 as 3 and 4, and all the other bays remained undetected. Seems something with hardware disk mapping is off. I am assuming now that the automatic script did not set my bios number to M911, and therefor the .pat file of DS1517+ updated my BIOS from M111 to M405, which is the version shown cat /sys/devices/virtual/dmi/id/bios_version
Leading me to this line in the opening text:
Do not update the 1517+ BIOS! It will lead to brick. Downgrade is not allowed.
Now it seems christian was able to somehow update the bios to an older version by tricking it. Will have a look into that.
UPDATE2
Using the opening post I was able to get back from BIOS M405 to BIOS M111 by renaming Bios M111 to M911 and doing the flash, after that all my disks are detected correctly again and I am still running DSM 7.2. If anyone ever encounters this, it seems my synology was not bricked by updating the BIOS to presumably the DS1517+ Bios (M405) but it detected disk 1 and 2 as disk 3 and 4. I suggest to not run the automatic github script and do it manually using the opening post.