Some good maths here on regarding why you don't want a shallow hill:
https://cyclinguphill.com/everesting-cycling-mount-everest/
"An average gradient between 6-10% would be best. You want to avoid something fairly flat, Cragg Vale would take ages because it’s only 3%.
Cragg Vale is 286 m height gain. But, 9.1km long (lap 18.2 km) 30 * 18.2 = 546 km
Hardknott Pass at 30% – 30 ascents of the 298 m climb. Done
Great Dun Fell – 635m – just 14 times. Shame about the two gates. Because that Everesting would require 54 jumps over gates in the road. Done!
The Rake – 101m – * 88 ! – But, a max gradient of 23%
Horseshoe pass – Wales – 311 m * 29 average gradient 5%, max gradient 10%"