Kudos to the software Carbon Copy Cloner. Everything is as before and lost only 4 weeks of data because of an old image (but that's okay, not super important).
Lessons learned:
- Test your backup strategy and your backups !!
- External APFS or HFS drives cannot be read by Linux (especially if encrypted with FileVault). Maybe I will switch to Veracrypt + NTFS (or FAT32) for external drives in the long-run.
- Restoring in recovery mode from a sparseimage did not worked out. A real 1:1 copy on an external drive is better and immediately bootable.
- Installing macOS Mojave on a bigger USB stick is slooow (3 hours?), the same goes for booting, but it seemed the only option for me to use Carbon Copy Cloner to restore to the internal ssd.
- Do more regular backups
- Maybe monitoring the internal SSDs health in future? The software drivedx looks interesting.
- side note on Linux: pinch and zoom gesture on trackpads is totally non existent on Linux desktop (even Windows is better with this). There are not even interfaces for it as far as I've seen.