Have ASUS ROG Strix B550-I, Ryzen 7 5700G and Team Extreme Memory. See if I can make it fit in a SilverStone SG-12 case. Optical drive, 3 1/2 inch hard drives and Seagate FireCuda 530 SSDs RAID0.
ITX
SG12 is just a SG13 that's stretched to look like an SG08
should be no fitment problems. More attention should probably be focused on your choice of CPU cooler - 5700G runs relatively cool for what it is, but does still run a bit warmer than the 4650G and 5600G. You can fit a NH-L12S in that case though, I use a NH-L12 Ghost S1 on my 5700G and it handles PBO and a 2300MHz iGPU just fine, albeit with plenty of airflow from 2 x 80mm fans. Will be choked for airflow by the overhead PSU in the SG12 so thermals idk.
Should note, however, that the B550 Strix ITX has only 1 PCIe 4.0 slot. Firecuda 530s are 4.0 drives - the one that you put in the slot on the rear of the board will only run at 3.0 x4 speeds. If you want full PCIe 4.0 performance on both, get a X570 Strix.
Though on second thought, a 4.0 drive stuck on the back of the board with 0 airflow will probably throttle under heavy storage I/O. Off the top of my head I can only think of one ITX(ish) board that doesn't have this problem (Impact) but it's a little expensive for a HTPC.