ITX has much more support than mATX. It's the main contributing factor to why I went ITX. I considered the mDTX Asus X570 Crosshair VIII Impact, but the prohibitive price turned me off.
There's ITX cases that accept ATX power supplies. My SilverStone FTZ01-E is one, but I opted for an SFX PSU because reviews recommended it for extra cable management room. Going for an SFF build is kind of backwards compared to a traditional ATX build. You need to settle on a case then pick compatible components. Decide what's important to you in terms of volume, footprint, and features and get a case that fulfills all that for a price you can afford.