Most competitive Everesting attempts use a steep and typically short road. 3-5 miles of downhill on a 5-6% gradient is probably too time consuming (i.e. too much time spent not ascending).

Ronan McLaughlin, the current mens' Everesting record holder, used a 12%, 800m long segment:

https://www.strava.com/segments/876936

IIRC from a video I saw about it the prevailing wind is up the valley, so he got a bit of a tailwind on the ups but the headwind on a 12% descent didn't matter.

https://cyclingtips.com/2021/03/ronan-mc-laughlin-has-smashe...

The steeper the better, so you dont waste time coasting or softpedaling or lose velocity to air losses. Steepness is also good for the descent, getting you up to a good (terminal) velocity.

It also helps if the segment is straight such that you dont need to corner.

IIRC the ones who were competing for the record didnt make the effort/recovery trade off consideration, their limiter was the segment.

I dont think they've found a longer perfect segment yet apart from the one in GB.

But from what I#ve seen the optimal ratio was 3-4/1 asc./desc.

You are then trading wattage for time. And given it's already a veeery long challenge counted in hours, it may not be worth it to make it even longer.

5% is way too low. 9 or 10% is about the sweet spot I'd say. But you need a road you can descend on with minimal effort too.