Do you mean in the "fill all the 20 slots" or in the "I have unemployed pops, enough housing and amenities, but have yet to unlock the next building slot" sense?
From the rest of your post it seems like you're talking about former, which I disagree with and I think the latter is the right approach. Because if I want to build something on my science world, it's because I want more research (or need more housing/amenities for my researchers). If it doesn't then I might as well resettle those pops to make more alloys.
Early in the life of the colony, it's good to alternate between the immediately-needed-resource districts, the science lab and housing, but later it's only housing and science labs, which supports at least 5-7 pops but only gives 3-4 jobs. It's possible to build more housing district in advance, but it's inefficient (as get more empire sprawl for no gain. Yeah it's not that big of a deal but why increase it when you don't need to?) and it just push back the issue anyway. Upgrading the science labs make the best use of those pops. Waiting for all the building slots to be used before upgrading, you would end up having dozens of unemployed pops and even with the living standards where they produce unity it's not as good as having them do a real job.
you could use them either way. i would argue that you would be better off trying to avoid using upgraded buildings in the ¨need to get more jobs to unlock next building slot¨ way because you would be better off building a resource district. But once your population starts edging past 80, you might want to start upgrading some because you have run out of other ways to give your population jobs, that i think is the best way to do it.
it is my opinion, that the best way to handle your economy is not to upgrade at all, until you have run out of districts and building slots and need to get more jobs.
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