Yes, problem solved.
I've redone all the process from scratch: unplugged power cord, removed the MB battery and let it removed for several minutes (> 5min), reinstalled the Ryzen 3600 CPU and boot with a single memory module first.
And it worked!
Apparently the first time I removed the battery only for a few dozens of seconds which was probably not sufficient to clear the CMOS. Thanks for your help, I'm now a happy user of a Ryzen 3600