So, I arrived to Gleba. To give myself some time to adapt and learn mechanics, I disabled Raiding mode for a while.
Then after the first artillery setup, I was faced with some absolutely crazy siege from the pentapods. 6 turrets + my actively fighting character could barely hold back the nests that were created all over the map.

After enabling AI point printing, I saw the culprit.
Nest killed: +190 points or smth like that.
Siege nest created: -200 points.
Revenge squad: -40 points.
Total points: 40k.

Result: if Raiding AI is disabled, after killing a few nests the player is forced to deal with the new nest waves for a LONG amount of time. Also, lag spikes.
And if Raiding is enabled, it draws the points from the same pool and helps stabilize the situation a bit faster.

Maybe add some warning about enabling Siege without Raiding or balance AP gains around enabled AIs to avoid "overfeeding" Siege AI? It just becomes tedious.

It feels like you're describing the Factorio 2.0 version of Rampant. There, both settlers and attacks used "action points"
But in Rampant for Factorio 2.1, the settler creation system was significantly overhauled. Sieges and settlers now use "settler points" and "siege points". That is, the presence of attacks does not in any way affect the number of nests created

I do not recommend using artillery. Using artillery is extremely dangerous for the player. Destroying nests with artillery accelerates the spawning of new nests. They'll first occupy the map far from the player's base, then begin to settle closer. And the more artillery is used, the more aggressive the planet becomes. This effect is permanent. Yes, it's possible to survive, but you'll need a thick perimeter.

If that's not an option, you might want to disable the Rampant on Gleba. You can also build "Level 3 Aggression Suppressor" to prevent the evolution from increasing. Before the appearance of the large pentapods, the attacks are significantly less dangerous.

Artillery can also cause lag even without Rampant. I haven't tested it in 2.0+, but in 1.1, Factorio couldn't support more than 3,000 biters. I'm talking about biters "on the move", attacking a close target puts significantly less load on the game.
This is one of the reasons why the biters in Rampant try to go underground on Nauvis after an artillery barrage. But on Gleba, this option is disabled, as "underground pentapods" would look too strange

Man, I'm starting to really appreciate your optimization efforts.
I guess I'll try my next playthrough with full rampant.
Out of interest, is Arsenal recommended now to balance new fancy biters? Or does it not play well with Space Age?

For example, how is the player expected to deal with the singing worms without the artillery? their range of 48 sounds unbeatable otherwise

And thanks for the explanation. That's why the pentapods became so aggressive after 2.1!

Artillery can also cause lag even without Rampant. I haven't tested it in 2.0+, but in 1.1, Factorio couldn't support more than 3,000 biters. I'm talking about biters "on the move", attacking a close target puts significantly less load on the game.
This is one of the reasons why the biters in Rampant try to go underground on Nauvis after an artillery barrage. But on Gleba, this option is disabled, as "underground pentapods" would look too strange

But doesn't Raiding AI also cause groups to traverse large distances? Or is it different from killing nests?

Could you also please clarify about the underground attacks?
The performance benefit is now obvious.
But does it mean they will be able to teleport through large bodies of water and the perimeter must never rely on them?

Could you also please clarify about the underground attacks, please?
The performance benefit is now obvious.
But does it mean they will be able to teleport through large bodies of water and the perimeter must never rely on them?

If there were no turrets in their way, they could. But radars and turrets force them to surface within a certain radius.

Out of interest, is Arsenal recommended now to balance new fancy biters? Or does it not play well with Space Age?

Arsenal is recommended. It's possible to win without it, but it will make the game much more enjoyable

For example, how is the player expected to deal with the singing worms without the artillery? their range of 48 sounds unbeatable otherwise

Combat power-up. It grants infinite ammo, such as infinite nukes. If you play carefully, you'll get more than you spend on destroying nests
Spidertron, under direct player control, is also capable of destroying large clusters of nests.
Furthermore, only one type of worm—fire worms—has a range of 48. The others have a range of 40, and that's only at level 10

In general, the same methods work as in vanilla, but with the caveat that you need to choose your destruction method based on the nest type.
For example, in the midgame, it's best to destroy Fire worms with a rocket launcher (with quality weapons it will be even easier) under the cover of laser turrets. An energy shield is essential. Another choise against fire faction: tank + poison capsules