Have Not really seen any biter expansions while playing, playing with only lightly tweaked settings and without mods that should conflict with the behavior.

save file attached if you want to verify im not just nuts
https://www.dropbox.com/t/N6UFfALMWwfmuvta

Hi thank you for hte provided save! I tested it. It worked.
But the changes are marginal. Expansion is slow regardless (2.5–39 min between attempts), so it's easy not to feel. If u want it visceral, high is the lever.
Messured settings:
Cooldown ~1.5× more frequent
Group size +20–30% bigger
Distance +1 chunk farther