Was facing the same issue on a 3700U with Vega 8
Was able to get it working smoothly on Arch using a patch
https://github.com/pioto1225/evdi/tree/amd_vmap_texture
Installed evdi-amd-vmap-texture from the AUR and edited /etc/modprobe/evdi.conf like mentioned on the GitHub to get it working
Hope this helps