Hi Guys,
as stated before, I come back (with very good news ;))
I follow the advice of @rodia and I did a firmware downgrade of my vontar X3 box to the april one (using linux because the card burner tool of windows didn't work as expected - it formatted my sd card to exFAT than Fat32) and now I can boot on Armbian bullseye 5.9 using the meson-sm1-hk1box-vontar-x3.dtb (thanks to @sag).
After this step, all goes well and now I have a vontar X3 with a armbian installed on my EMMC, 1Gbs full-duplex ethernet, wifi and BT fully working!!!
I hope that this topic will help one day someone else.
Again, a great THANKS to you guys, armbian and this forum!!
Antoine