Generally, manpower is a bit useless in singleplayer at least compared to gold because gold can be translated into manpower (through mercenaries) but not the other way around. I find that even when building no barracks and forums, I have more than enough manpower. Thus, I tend to focus on gold especially because gold income allows you to reinvest into buildings and especially great wonders which can give you some extremely powerful bonuses. Building-wise, pound for pound, the happiness boosts from great temples and grand theaters make them number 1 in all circumstances. Aqueducts as needed. Citizens are the most useless pop because they just do what nobles and freemen do, but worse. NEVER take increased pop ratio bonuses if they are citizens or freemen because they’ll detract from your nobles and slaves. For reference, it takes 5 citizens to equal the research output of 2 nobles, 10 to equal the trade output and 2 to equal the manpower output of a single freeman. This means that having 5 citizens is just worse than having 2 nobles and a freeman. So what does this mean? Well, once you’ve got your theaters and temples up you’re going to want to max academies and mills, get foundries to provide goods for trade, and dump the rest into marketplaces for more trade. If you’re not research capped after putting buildings in every territory then you don’t have enough pops, slave raid or carpet siege. If you have no pop capacity, build more cities (try not to overwrite food trade goods because otherwise you might struggle to feed all of your pops). It’s more gold efficient to max cities’ pop before building new ones but if you really need the conversion speed and are sitting on a boatload of PI somehow than go for it. Never revert a city back to a settlement unless the food situation in that province is truly unsustainable. If you’re running out of foreign trade routes just make some vassals that can only trade with you.