When I get to my 6th planet, my first colony is moving along at maybe 10 pop. my colony numbers 2 and 3 are getting to 5 pop, and colony 4 is just growing pop2 or 3.
So when I must make my first Sector I usually have to sacrifice my first colony. Currently the AI prioritizes pop growth by building as many farms as possible and removing blockers. While selecting not to redevelop existing tiles helps, the selection to respect tile yields is ignored. With no redevelop selected, all those farms it builds are stuck as farms after the pop is full. But if you remove the limit on redevelopment, it is really stupid. So if you have a fully developed planet, the sector can maintain it. This is supposed to be fixed.