Have you tried Thunderbolt accessories? You should be able to add NVMe storage and AMD graphics that might speed things up. I couldn't try this out because I

Have you tried Thunderbolt accessories? You should be able to add NVMe storage and AMD graphics that might speed things up. I couldn't try this out because I have an nVidia eGPU, but using Apple's TB2-to-TB3 adaptor I can actually game quite acceptably in Bootcamp.