5900X and RTX3080 are compatible. Only some CPU with hybrid GPU structure may interact due to drivers, but that would take advantage in DX12 games.
AMD 5900X is just a CPU. Any GPU will do. My son got AMD 3900X CPU with GTX1660 GPU. Super performance and no issues.
RTX 3080 is really solid GPU. I got RTX 3070 and no problems at all.
Anyway both your CPU and GPU are ones of top notch.
I wouldn't trust AMD GPU display drivers, also overmarketed and overbenchmarked 6800XT is so great only due to DLSS equivalent (FX functions) to blurry all for performance and sharpen the objects in focus again for quality. No need to pair CPU and GPU brands. You are good to go.
Performance wise, such setup will be excellent and far ahead of time !
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Now I would like to steal the topic. Same theme.
I got many modules on Steam linked with ED account.
I want DCS for my son, but Steam sales are over. Beside of shortage of good HOTAS in UK, if he goes Standalone and we intend to play together, there is no limiting factor?