Hello, thanks for your answer.
Sure, of course I had seen that link but it misses a LOT of information.
I cant tell you how many hours of research and how many tabs are opened on my computer…
I found solutions to my problems :
My FP3 is definitely dead.
With a bit of luck I found a second hand FP3+ nearby, so I bought it.
Now I have a new phone , a dead motherboard and many spare parts.
About my FP2 , the problem was that TWRP was not properly installed, flashing it allowed me to re instal stock android.
All this was a bit exhausting.
thanks again for your time.