Benchmarks run by the guys at Hardware Unboxed have shown performance falling off past the GTX 1060 on the Ryzen with Vega chips. The theory is that the normal 16X PCI-E lane dedicated to graphics is split in half on the 2400G with half the lanes going to the onboard Vega and half going to the actual PCI-E slot, and that this configuration is impacting performance.
I think the GTX 1660 Ti would be the better buy with the best chance of getting the most performance out of it.