I general I wouldn't let sectors build up new planets. I don't trust them with building placement. I normally find ways to have control of developing core planets, then queue up at least the base buildings. After a bit, I let the sector take over upgrading or anything else with redevelopment turned off.
I feel like that is the only way to play vanilla. I wanted to play a few games in Utopia w/out mods to see how things were working, especially in light of all the new elements involving core system increases. It really feels like cheating to re-activate my unlimited core-worlds mod, but ... the alternative is SUCH a slow game-pace, where I have to develop everything myself or eat my hat in frustration at the terrible development choices.
If sectors could properly develop a world (not OPTIMALLY, but just properly---because optimal development often dis-respects tile-bonuses) then I think they'd otherwise be a great element in the game, now that we have good options for increasing core systems to a reasonable amount through development mechanics. Maybe next year?