HAGs will be avaiable only if your driver is up2date. If you not see it, your driver is possible older than 20.5.1 beta.
(https://www.amd.com/de/support/graphics/amd-radeon-5700-series/amd-radeon-rx-5700-series/amd-radeon-rx-5700-xt)
BUT: at least for NVIDIA I can never recommend to enable it !
And… what is the issue with usage of 7.99vram related to a card wiht have 8Gig Vram ?