It's better to do follow/unfollow on the same day because then you never have days when your gains slow down. This way you just get a consistent gain of followers each day.
Depending on your follow back ration maybe around 180 followers a day?
What your niche is doesn't really matter, travel will grow faster than student but everything will grow
You can use 100k - 500k, but there is a pretty good chance you followers are going to be from all different locations around the world.
I only mentioned the 4k concept if you want to target local accounts!