I love the idea, this may be a great addon with some hard biter mods, how is compatibility with things like
Rampant (https://mods.factorio.com/mod/Rampant) ~ Armoured Biters (https://mods.factorio.com/mod/ArmouredBiters) ?
and / or Alien-Biomes (https://mods.factorio.com/mod/alien-biomes) ?

Compatibility should be a complete non-issue. The mod only operates on entity types, not specific entities.

If you look at my mod portfolio, many of them are designed with compatibility in mind and work well in a broad array of modpacks.

I LOVE and hate dangOreus ^_^

That is the highest form of praise. :D

Seems like a good way for "passive proactive defense" to get all the land once the fun of fighting biters has turned into a choregrind and a timesink.

How is it in early game? What's the balance like? Hard to put 60 spawns into perspective when most kills happens by turret off screen and I never counted the spawners. Will all spawners that touch my pollution cloud just die off quicker than I can claim the land in early game or will it only actually be noticable in late game?

60 spawns to kill probably means negligible impact. I let it run for hours on my save and I noticed a couple of spawner deaths after hours of letting it run. They show up in the kill tracker.

Is the UPS impact noticeable?

This has negligible impact on UPS. This is nothing like Peppermint Mining