So, past my Ryzen 3000 problem, I now tried to boot a Ryzen 7 2700... to no avail.
Either both memory slot are populated, and I'm getting a solid orange q-led (RAM issue), or a single slot is populated, and I'm getting a solid RED q-led (CPU issue). Once again, no problem with an Athlon 200GE. CMOS reset doesn't change anything.
At this point, I'm gonna try on a Gigabyte mobo. At the end of the line, either Asus or AMD is garbage.
As usual, any hints appreciated.
ps: my memory *is* in the QVL for the B450-i
edit: just for the sake of doing something, I'm gonna run some memtest during the day...