Hello all !
As promised, here are the results.
One week of meditation after, I decided to flash.
Installation was successful. If it can help:
- I downgraded from Q to Pie with the build XEF-G960FXXU7CSJ1 without any trouble.
- I installed TWRP and made a complete clean as explained here step 8.
- I then tried to install /e/ but stumbled on the famous Error 7, just as on the topic Archje mentionned.
- So I patched my device as explained by the wiki with the right vendors with ADB sideload.
- At least, I could install /e/ with adb sideload too.
I can confirm to you that /e/ is perfectly installed on a Android 9 base.
Thank you all for your help! 