Rampant is a fairly Popular mod so perhaps you have already had a look at it. If not, perhaps you could investigate if it's possible to support it. I'm phrasing this carefully because Rampant dynamically creates variants of spawners and I also don't know whether its siege mechanic, where biter nests specifically expand towards player structures, causes any issues. As far as I can tell it makes a couple changes to creep decoratives and spawners but doesn't come with any drastically different new types of creep or spawning mechanics, so maybe it's not as hard as I imagine but if it can't be done then that's fine too.

Perhaps performance impact is worth considering here too. Rampant tends to spawn A LOT of nests. I'm not sure how intensive creep cleaner can get in these cases.

The mod doesn't work well with any type of nests expansion (even in Vanilla, but to a lesser extent) because the decoratives are not attached to a specific spawner but rather its position on the map.

This mean that when you kill a nest, the mod mark an area at its position to be cleaned, and if by the time the area is cleaned an other nest spawns, the decoratives of the new nest will also be cleaned.
This may lead to nests (mostly around player structures with Rampant as you described it) to not have decoratives under them which may look weird, but it is not the most problematic issue there.
Like you guessed, performance will be a more problematic issue here. Because even if there isn't anything to clean under theses new nests, the mod will still mark the area to clean, piling tasks over tasks since you're more likely to clear the area around your base frequently. And as your base extend, the killing zone expand. As well as the area to clean.

Plus, because as if this wasn't enough, I'll just let you judge by yourself:
- Let's assume we're still early game, but mid early game.
- Now, let's take as example a "modest" enemy base, composed of 20 "spawners" (biter spawners, spitter spawners, worms).
- Let's assume each of them in average spawn 6 different decoratives types.
The mod already have to search and clean decoratives in 20 x 6 = 120 areas because each decoratives can spawn at different distances for each "spawner".
This is a lot. 10 bases like that, and you're already at 1200 areas to search through and clean.
And I'm just talking early game...

So, with a mod like Rampant (I may take a look at it anyway, but it is not a priority) you're just going to pile up tasks over and over that will most likely never going to be cleaned up because of your kill rate, inflating your save file (you have to keep track of theses killed nests right) and eating up UPS I'm pretty sure you would better like to spend on Rampant AI or your factory for example.

I hope you have a better understanding of how the mod works, and what you're asking now. ;)
(But don't hesitate to correct me if I got something wrong with Rampant).

With that clarified, the AI Siege behaviour is basically asking for problems. The AI "creeps" closer (pun intended), you clear the nests, meanwhile the AI is already advancing elsewhere. It's less of a problem once artillery is involved but the problem is still there. I'm already seeing UPS drops in sieges whenever I run out of Artillery Shells simply due to the sheer number of enemies coming at me. This siege behaviour is actually why creep cleaner got my attention because after a few hours of no artillery shells, your walls pretty much get surrounded by creep, But it's fine, it'll go away at the latest once I bring in the concrete.

Your Explanation also revealed something I hadn't really considered yet. The Rampant AI loves to repeatedly nest in the same areas (because it gravitates, for example, towards ore patches) which would only exacerbate the problem with expansion.

It's unfortunate but I can see why it'd be difficult. But as I mentioned, I expected something like this. Modifying mods to my needs has taught me enough about the game to understand that things are often much more complicated than they seem. I guess it could be done with a lot of effort and disregard for performance but It's probably not worth it for something that would just be covered in good ole concrete eventually.

Thank you for your time anyway. That explanation was actually pretty fascinating. Being a comp. sci. student, the exponential increase in stuff to clean hits home. Big O is strong in this one.

I'm glad you got something out of my rambling. :P

But yeah, I think too it's just asking for problems.
I've already optimized a lot the last version (0.0.23).
And beside changes to the internals of the game, there's not much left to do to (greatly) improve the mod performance.
Devs won't do it (because it doesn't make any sense for them to do it), but ideally, to greatly improve performance even more, it would help to know for each spawner which decoratives are attached to (spawned by) them instead of hammering their API to search for them.
But even with this, it wouldn't help for the Rampant AI Siege problem. At best it would just delay it.
So, yeah. Like you said: "Big O is strong in this one".

No worries for the explanation. My pleasure. ;)
And thank YOU for your feedback on Rampant gameplay (interesting mechanics), since I knew about the mod but still haven't played with it yet. ;)
Good luck with your studies!