Not exactly sure if its a bug or something you intended to have, but i've noticed that new immigrants are only going towards my first settlement
Yes it did. It had enough for around 70-100 people I built the whole settlement with the starting 17 people.
I even send in a second migrant wave from the capital after a few Years and started importing food from other settlements because the 17/34 people couldn't support all the houses I had built. It took 3-4 Years for the first real migrants to show up.
You should actually still have the save with that settlement it was the last I uploaded the settlement between the 2 mountain areas. Btw. you said you wanted to use it for performance test. Were you able to fix some of the crashes happening every few minutes?