No, manufacturers had to redesign their mainboards vor v5, because the frequencies got so high they had to move components closer to the CPU. That and heat dissipation and efficiency issues in first generation, which are still somewhat present on current gen.
At least a few months ago, recommendation for new builds was to go with a v4 mainboard.
Not for consumers. Anything beyond PCIe5 is basically just server stuff. Frickle in handling, and expensive like sh-t. Just the same as with PCs: You don't need a fat gaming PC for using office and browsing.