There is an option under Endless, that adds spawn time to the waves once past Hold Time. So if you have waves set to 2 mins, and want to have waves less frequently during Endgame, you could set Endless Delay to 3 mins, which would cause waves every 5 minutes.
There is also the option under Wave Multiplier to make Endless Waves smaller. The final number is the multiplier applied to the endgame wave. I believe I added some that go:
2 > 2 > 2 > 1.5 > 1
So if you had wave size set to 10 units, it would spawn 20 units most of the game, then drop down to 10 units for endgame.
Added those two options for players who wanted to be able to ground assault the HQ. Though, maybe I should add a defensive wave, only triggered by units near HQ after Endgame begins. Hmmmmmm...
Edit:
There is also a third option. Set everything to spawn from 2nd Spawn, with no HQ recovery. Destroying 2nd spawn would then eliminate all regular waves. Or set HQ recovery to 25% or less, which will keep units spawning, but in greatly reduced numbers.