The point of building this rig was to upgrade my sons current setup to something a bit better for his gaming needs. I'm reading that amd running in single channel isn't great for performance.Have you considered just putting up with one RAM channel? Especially if it's not your primary PC, and you get the performance you need.
I have the same issue with an intel B660 motherboard, and running a i3 12100F with a single 16GB stick with a RX 5700. Still works fine in games with that combo.
Single channel memory loses performance but not exactly sinful!
I wonder if there's any benchmarks comparing single channel performance vs dual? If 8gb in A2 and 8gb in B2 performs similar to say, 32GB in A2 then I'd consider keeping it for him. But my hunch says it will be drastically different.
Besides, I bought all these components recently, so I have time to return them as majority are from Ebay.