Thank you for your answer.
Two weeks, i used my 1st Archer C7 v5 during two weeks before bricking it (may be less).
My intention in the beginning was to try both open firmwares (DDWRT + OpenWRT) and eventually keep the one which would be the best to my use (i came from Tomato during several years and loved it - so i would have kept the firmware who would be the closest to Tomato - in term of possibilities, interface, etc).
After i installed DDWRT, i 'played' a bit with it and very quickly i wanted to try and see OpenWRT. Sadly, it was not possible to switch directly from DDWRT to OpenWRT, so i tried to revert to factory firmware, it never worked...
This led me to do things really stupid to try to revert the Archer C7 v5 to factory to install OpenWRT.
Firstly i tried to webupgrade to the oldest factory firmware Archer_C7_V5_190726.bin, never worked, then I tried several tftp firmwares on PC and MAC,on PC it never worked completely (i always got a ack error during transfer - i still have the log) and on Mac with PumpKin the .bin were sent each time, but after the router restart, it was always on DDWRT.
I can't count the number of times i was following a tutorial, or a guide thinking it was for C7 v5 and it was for the v2 v4 or even Archer A7, it is really easy to make a mistake router reference are really close and not every doc/guide/videos are showing the router revision.
Anyway, hopefully the Archer C7 V5 is not too expensive, and i got another one ...
And By the way, i saw two different way to install OpenWRT, the 1st from TFTP, and the other from Webupgrade, are both working (I prefer Webupgrade, tftp scare me)?