That's why you basically have 3 types of planets:
Manufacturing planets - nothing but factories, a starport, and wonders. You don't need farms or economics or morale enhancers. There are no multipliers for manufacturing (except for starbases and your manufacturing capital).
Economics planets - your best PQ planets. You have farms, stock markets, and entertainment centers. This is what high-PQ planets get used for : cash cows. You need 3 types of improvments to have a good economics planet, and that's why you need PQ here.
Tech planets - Those 4 PQ planets make good tech planets. All you need here are research centers. Unlike manufacturing and economics, research can be spread out over lots of different planets. The only exception is your tech capital, which should be a high PQ planet.