AMD Graphics Driver Manjaro Linux 20 Setup – Step-by-step Guide
How to Install AMD Graphics Driver on Manjaro 20.x GNU/Linux – Step by step Tutorial.
And AMDGPU Pro for Manjaro 20 includes the AMD Proprietary OpenGL, OpenCL, and Vulkan Drivers.
Especially relevant: to Check if your Board is Supported see AMD Graphics Drivers Software Compatibility list.

Installing AMD Radeon Graphics Driver on Manjaro 20 Driver
How to Install AMD Radeon Graphics Driver on Manjaro
Amdgpu Pro Driver Manjaro Linux 20 Setup Guide