With maxed out trees, woods so dense, so:
1) it's hard to walk through even with reduced collision
2) trees have priority over rampant => no biters spawn in woods. Like, probably one worm spawned and that's all, no nests

Are you using vanilla biter generation?
Thats what you should switch to for rampant.

And RSO doesn't touch trees in any way so.. you maxed out trees you get no enemies then?

• Rampant bitter generation; RSO vanilla bitter generation disabled, RSO bitter generation disabled
• There is enemies. They spawning, migrating, everything seems okay... But if enemies trying to spawn in woods on game start, then they fail to do that. Like.. One spawner instead of six; two worms and zerospawners; if hive should be on ende of forest, then only "woodless" half will be spawned... But everything okay on sand land

There is day 0 new map generation. Woods 100%/150%; Bitters 400%/200%; Starting zone 75%
https://ibb.co/QfxQpSq

What is the rampant biter generation?
Since you disabled both vanilla and RSO built in generations then you might need to report this to rampant.

My bad. It was vanilla generation without RSO generation.

RSO generation without Vanilla generation successfully ignore woods on first glance.

From what I recall rampant had issues with RSO generation.
And if vanilla generation ahs problems with trees that might be something for base game.

RSO: successfully spawn wrong bitters
Vanilla: successfully spawn woods and sometimes correct bitters

I think, you can't fix anything here. Thank you for replies!