I think the best way to make one such system is to depend on systems that other such games\modes have used:
- Killing Floor: In mission weapon changing mechanics, here i would suggest the same mehanic that I would for Darktide’s mechanic
- Gears of War’s Horde 2.0: Having special buildable in mission to help out.
- Progressive loot return, giving the equivalent in the resources.
I think that’s the bad way of doing those things, people like to see number go up, so having the waves be faster, and have short intermission time.