Except this is pcie 4 vs pcie 3 so it's double the lanes for the latter, so no, x8 pcie 4 will not bottleneck anything. Unfortunately GPU do not support pcie 4 yet, but it's not a problem for integrated 10GbE.
Yes, the Navi card of course supports gen4, and even the Vega20 did too. At least in the original Instinct variant (most places on the internet say that gen4 was cut on the consumer Radeon VII card)
