Does anyone know the rules for base raids?

Its been well over 100 game days since the last time i had a raid on my base. And ive only ever seen the 'forest is moving'.

Right before killing the 2nd boss i built a new base, got 3 raids in quick succession while building it. But ever since then i killed the 2nd boss, did all of the swamp, killed 3rd boss, did all of the mountain content. Havent killed 4th boss, but could at any time, essentially done with mountains doing other things atm.

Starting to get annoyed i haven't seen any in a long time! And annoyed I've never had a higher tier raid then the first one.

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At first i thought you needed a bed to trigger a raid. But then early on i got raided without a bed. Then i was thinking that you need to stay in one spot for a certain number of days....but I've done that as well multiple times smelting up a boatload of ore and chopping mini forests worth of planted trees just outside my base to fuel the fires of isengard. Have smelted up more then 3 full longboats of ore since then, so spent plenty of time standing at base.

Anyone have any idea what the trigger conditions are?