Your main manufacturing hub will always be Earth
Whoah, slow down there... You'd be surprised if you saw my playthrough.
I would agree, you should never focus industry on any one place... you should make sure that minerals are the thing you move the most eventually unless it is a new colony. Most well established colonies should have industry too support themselves so all you have to move in space are minerals. Minerals take up such a small space in comparison to the final product. You will save both minerals, population productivity and fuel this way over time.
Thing you can focus heavily on at Earth would be population growth, Wealth and Research... I would usually offload ship construction for Luna. Earth only retain enough industry to support its own population growth and some extra.
Anything that is a physical thing you should spread out construction off as much as possible, the more population you have on a colony the more self sufficient you should make whatever items they need... so this is anything from facilities, troops, supply, fuel, missiles, fighters etc..
My colonization plan are usually to make any colony self sufficient in terms of industry by the time they hit around 100m people... depends slightly on how mineral rich that colony is. By the time a colony hit about 300m people I want them to be completely self sufficient on pretty much all needed production long term.
Of course you still have some colonies being more concentrated on some things than other, but for me that comes after they are self sufficient. Smaller colonies often are very focused on one or two things. There also are the possibility to have one large production center per system. If distances to colonies are small enough then using freight to move facilities are not too expensive or time consuming.
Cargo ship who transport facilities should mainly be relegated to Auto mines for none colonized worlds and to kick-start new colonies. Your core colonies should only need mineral transportation to do its work, it will save you allot of resources (even when you factor in governor bonuses for specialization).
The most important thing is to make sure you have a balanced income which can sometimes be hard in this game.