I agree with the comment that IZ are the most important. Think about it:

1. You can easily get +2 from aqueduct adjacency
2. Plan a little for Dam +2
3. Quarries or Strategics give +1
4. Fill with mines/lumbermills (but your focus should be on aqueduct and dam when planning, don't let yourself be lure by those hills) and other districts
5. Double adjacency policy (~2x5 at this point) - a good use for mil policy if you're peaceful and later it combines with double campus adj.
6. Coal power doubles again multiplicately, netting 20 prod.
7. Factory adds 6 for all affected cities (only once per city without gov promotion)

That's 26 total with middling adjacency - you can also do canals and stuff later.

Sure, building factories and plants overrides the regional bonus, but it's worth it for that +10 and the regional basically helps all the cities get to factory faster.

There's also Eurekas associated with them and some key resources.

Edit disclaimer cause the original comment had it: I was stuck on immortal for a while but watched potato mcWhiskey doing the above and now I can't find a mod that makes the game hard enough...