Hey guys, i finally resolved it.
I'll write exactly what I did, but there will be steps that are possibly unnecessary and/or unique to my case:
- restored from NAND backup of "1.0.0-7 pre-release"
- generated "1.0.0 official" offline firmware file via eMMChaccGen and prod.keys file from yuzu, found via google
- flashed the generated 1.0.0 files to my switch via tegraexplorer by injecting tegraexplorer.bin file and executing the systemrestore.te
- then I mounted emmc once again via memloader.bin to my windows system and opened the disk via HacDiskMount
- killed all the files from SAFE, SYSTEM and USER and replaced them with the files from offline 1.0.0 once again, also removed the PRF2SAFE.RCV files in all three partitions
After this, the switch booted via hekate 5.3.2 and AMS 0.13.0 and is now fully functional again.
Thank you very much for all the help and hints you gave me!
But in the end, I had to find my very own way and it was worth investigating for four entire weeks..
Never give up!