It's like reality people stay where they lived for the past 30years.
As far as I know, reading nearly everything here, it's not possible to do a mod like that.

That's indeed not possible. People are made to always try to get into their own home, as a couple. You'll sometimes see couples of widowed people get reformed, freeing a house, but you can't make someone who has a personal home move into a common housing thing like the boarding house.
And based on that, the people in the boarding house will also always move into a home if one is free.