Solved! Here I am, writing again from my EOS install on my laptop. It has booted.
And besides, I have found out how to permanently deactivate automatic updates on Windows, so it doesn’t shoot my bootloader again on the evening before an important day on which I really need my laptop to just work 
Thank you so much again 
EDIT: Thank you very also to @clearmeth for the description of how to get systemdboot back in order.
This thread shows well two of the reasons why I have been staying with EOS as my main OS on all my computers since 2021:
- The great, suppportive and friendly forum
- the possibility to do with my computer what I want/need when I want/need.