The 6800XT won't likely have reviews available for a few days still, so it's hard to say exactly how it will perform, and whether or not there have been notable improvements made to things like the video encoder. Some things, like raytracing performance, may also require optimization by game developers before we have a clear idea of exactly how they will compare to Nvidia's offerings in the long-term.
And based on rumors, the 3080 Ti isn't expected until early 2021, so it's probably still at least a couple months away from release, if not longer. Logically, I would expect it to perform somewhere in-between a 3080 and a 3090. Since a 3090 tends to only be around 10-15% faster than a 3080 at 4K, with even less of a difference at lower resolutions, I would not consider the 3090 at all unless you have money to burn, and don't care that a card available in a couple months will likely offer relatively similar performance and a similar amount of VRAM for around two-thirds the price.
And even the 3080's 10GB of VRAM may not be as much of a concern as some people are making it out to be. Currently, it doesn't tend to make much of a difference in today's games, even at 4K resolution. Maybe a few years from now it might impact performance more in some games designed around the new consoles, but if you are already replacing a 2080 Ti, it's possible that you might also replace this card before VRAM has a chance to really impact performance. And if you plan on gaming at 1440p or below, VRAM should be even less of a concern.
As for the CPU, if you were gaming at 4K with ultra settings, then most demanding games would be limited by graphics performance more than anything, and you might not see that much of a difference between a 3900X and a 5900X in most titles. At lower resolutions, it could make significantly more of a difference though, and on a 240Hz 1080p screen, it would likely make more of a difference than the graphics card upgrade in many titles.
So if you are gaming at 1080p 240Hz and/or 1440p 144Hz, the 3090 probably wouldn't be worth considering at all, and the same likely goes for the 3080 Ti. There's only likely to be a few percent performance difference between a 3080 and a 3090 at 1080p in most games, and not much more at 1440p, as performance will typically be CPU-bound more than anything. The jump moving from a 2080 Ti to a 3080 would be larger, though not exactly huge. Again, especially at 1080p, performance will often be CPU limited, at least when targeting high frame rates. For something like Minecraft RTX that makes heavy use of raytracing though, you would likely see a lot more of a difference moving from a 2080 Ti to a 3080, since the frame rates will be almost entirely graphics-limited. In something like Warzone though, I would expect the CPU to make the most difference.
So, of these choices you provided, the 5900X + 3080 might be the most well-rounded for your usage scenario. The 5900X + 6800XT might potentially be a good option too, though it's hard to say for sure quite yet. We will likely have at least a somewhat better idea of how the Radeon RX 6000 series performs next week, once reviews start coming out.
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