I ran into another issue in my play-through: big stompers arrived (in numbers) before any behemoth biters showed up, and while I can handle biters, probably the only way I could kill a big stomper with my current factory is by placing down around 100 turrets and accepting losses. But given that they keep spawning that's unsustainable.

It's sort-of acceptable, forcing Gleba to wait for better tech... except weird that I only ran into this problem much later. (Probably partly because initially I had biter nests spawning there, especially armoured biters).

In the stock game each planet has its own evolution. I'm not sure if that's fixable here. So maybe this is unsolvable.

Working on a fix now.

Version 0.12.2 has been uploaded. This should resolve the issue... of newly spawned pentapods.

But you seem to have many of the big stompers now. They must be killed, to be replaced by the weaker pentapods.
As a suggestion, install my mod Planetary Peace, then enable Peaceful mode, this will kill all of the enemy units over a small period of time, the spawners will then spawn new units. Then click again after a bit, to return to hostile mode. You can then uninstall the mod if you like.

Edit: I just updated the mod, the animation is confirmation the active units were removed.