I did try to fix booting with this procedure : GRUB/Restore the GRUB Bootloader - Manjaro.
No succes, despite well preparing the commands in a text file, before starting Manjaro live from usb drive.
Too much to explain what went good (a lot) and what went wrong … 
I can still booting into windows, which makes me this time more happy than ever 
Next trial : find another manual with the proper procudere … 
Searching for “Hoe herstel ik het opstarten van manjaro linux op een laptop met een ssd disk ?” = “How to repair booting of manjaro linux on a laptop with an ssd drive ?”
So nice to ask it to “DuckDuckGo AI Chat at DuckDuckGo”. You guys dont like AI ? I will take care 
The answer provides a lot of knowledge to use for finding a real manual. Which I do need. Gambling around is forbidden in this quest !
Will check out again : https://forum.manjaro.org/t/root-tip-recovery-basic-manjaro-linux-rescue-and-recovery/175302
And this one ? Small but good ? https://jrehkemper.de/content/linux/grub/manually-boot-from-grub-shell/