Have you tryd rocm driver?
found this page (maybe helps)
https://www.phoronix.com/scan.php?page=article&item=radeon-vega-setup&num=1
also amdgpu pro 18.10 and 18.20 suggest to use vulkan 1.1.70.0 (latest 1.1.73.0). hiveos has 1.1.60.0
also link comparing gpupro and rocm (old driver )
https://www.phoronix.com/scan.php?page=article&item=amdgpu-1740-rocm&num=2