Very good question. I do not know this, too. I was asking myself the same.
On Asrock-website for A300:
Memory - Supports 2 x SO-DIMM DDR4, Max. 64GB (non-ECC)
It is means ALL RAM without just non-ECC or if connected the maximum of 64GB?
Perhaps "(non-ECC only)" or "until 32GB ECC support" is more meaningful!?!?
The question is, is the mainboard compatible for ECC?
So, A300 has got full 72-Bit (64Bit Data + 8Bit ECC) and UEFI-Support, too?
The CPU Pro-Series can control ECC-RAM.
And X300? I see no differences of A300, just UEFI-Firmware. And that USB 3.1 gen1 is called USB 3.2 gen1, now. <-- it is the same technic, speed, port!
In BIOS hmmm that is no BIOS, it is UEFI... It could written "ECC", but take care. The original motherboard could have ECC, but A300/X300 is just a hmmmm lower revision from a PC edition. And some Linux and Windows users had troubleshooting with Windows and lagging YT-Video and CMOS crash because Crucial P1-NVME + Linux Mint, because the UEFI-Firmware (P3.60[final]) of the Deskminis is not perfect. As like as, the newest final A300-firmware (P3.70) is with older AGESA as the previous BETA versions (P3.60K).
For renoir CPU, you will need: AGESA V2 1002 !
That is:
A300 - P3.60S (not official)
X300 - 1.46 ( I do not have X300, perhaps 1.40 is ok!)