I personally have lots of positive experience with Linux [and Windows] on ASRock B450 boards [with Zen, Zen+ and Zen2 Ryzens, including APUs], and specifically the ASRock B450 Pro4 [ATX], ASRock B450M Pro4 [mATX], and ASRock Pantom Gaming mITX… and have no hesitation recommending them.
I posted some details here (with some uptime data).… These systems have been very stable for me…
As @wertigon stated, be very careful with B550 boards, as officially B550 does not support the 3000 series [Zen+] APUs…