Hello,
I had a lot of trouble (even fell into Qualcomm crashdump mode…) but finally managed to install back Android 11.
I found capital information here : https://droidwin.com/unbrick-oneplus-8-pro-8t-msm-download-tool/
and here : https://www.youtube.com/watch?v=5_40-jElRP4.
And here : https://www.youtube.com/watch?v=5q-6yM6BfZs
It turns out my phone needed an International OS (and not European).
Not the first time this happens : Recovery image for one plus 8 pro - #14 by Nello.
(Still unable to use Easyinstaller though (although I installed the latest version of Android 11), I’ll create another issue…)
Thanks for your help !