@NightHawkRMX It's pretty much the same conclusion I arrived at after scouring PCPartPicker and 10+ motherboard website CPU Support listings. I either spend $160-200+ on an X570 that supports Ryzen 3000 (the market is a bit high right now) or I update the BIOS on a B450. Which will (usually) require an earlier chipset to complete the BIOS update, which means I'll need a temporary CPU from the Motherboard manufacturer, or I'll try to have it done at a local shop (which also varies in price and availability).
However, you can use a Ryzen 2400G which is compatible with mostly every B450 MOBO; it has Vega 11 Graphics (just like the 3400G), and you'll only suffer about a 5-7 FPS loss on most titles. So if you're in need of a cheaper Mini-ITX B450 MOBO for a portable build and don't mind the FPS loss, that's the best choice. You can get them used on eBay for about $120-130.
I posted because my friend has a spare 3400G and wanted to make a portable LAN build, so I was trying to shortcut the research process by asking.