I REALLY wanted to get a 6800xt or even a 6800 to replace my old vega 64, and like you I think I was influenced by the power of CONSOOMing. Now that supply has been out for months I'm thinking of just waiting another generation. The two most played games for me this year so far are Star WArs Battlefront, the OG from 2004:
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And Sins Of a Solar Empire Rebellion
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Neither of which require a supercomputer to run.
The R7 cards are all GCN. They support DX12, there is no reason they shouldnt run either RDR2 or HZD. When they say "older AMD cards" They're referring to Evergreen, EG the HD 6000 series and lower, which are unsupported.
What card do you have? It may be time for a OS reinstall, could easily be a software issue instead of hardware.
Vulkan is only used in a handful of titles, and most of those were from iD, who is now owned by microshaft. So I wouldnt count on vulkan seeing widespread use anytime soon.