I am hardly an expert.. But I have been grabbing any resource i find early, and just placing one constructor on it. This gives me a slight bonus, and keeps the AI from getting one.. the ones near by, i upgrade, the ones farther away, i risk.. If it falls later in a war, then so be it.
As for building planets, lately i have been making most planets, a single starport/factory/lab, then a few economic buildings. If the moral is low, i build a moral booster, If it has influence based tile boosters, i build those..
this lets the planet slowly build ships, or constructors, and in times of war i can focus on ship construction, to build things a bit faster.. when the planet is not building tile improvements, and i don't need ships, this lets me swap focus to research.
My research and factory capitals i try to find worlds with tile improvements in that area, my home world i make my economic/influence capital, the secondary planet in my home system i build all the stuff that gives bonuses.. but they take a long time, so i build some extra factories. this also helps to pump out colony ships and constructors early game.
perhaps some of this advice will help you.. as i said i am new.. maybe have 75 hours played.. I also posted a thread about a week or so back that has some nice info..