Since nobody discusses this topic
I looked at it a little more and I think that going for the discount of the first two districts of each type is a no-brainer because it's so easy to get.
You only need "a +1" districts to unlock 2 districts per type. That's insane.
If you don't go for a religion, delay Astrology as long as possible. So even after unlocking IZs, a = 6. You only need 7 districts to get 2 IZs discounted!! Or at that point, only 13 districts to get the discount for 3 of each type !! How is this not more discussed here?
I don't care about the drama!
Also, in my last game I wrote down the production costs everytime I've waited for a discount and even after 30 turns, it was still a lot cheaper (only 2/3 of the original costs by pre-placing depending on research speed). So I always ask myself now. Do I really need another commercial hub right now or do I build a cheap theatre square for example instead, grow the city and place the CH at a discount later? Because by doing so, I get two districts for the price of 1 and unlock more discounts again.
This concept can be applied to many situations in the game where it is better to wait for another district to finish because it then unlocks another discount for the one you originally wanted to pre-place. By doing so, you can save a lot of hammers. Just wait and build something else.
It's quite interesting. And fun because you actually have a motivation to build a couple of camps and entertainment districts as well. Camps are dirt cheap anyway with the -30% policy. Add Liang and Merchant Republic for a good laugh.
By the way, Germany is insane at spamming discounted districts. It never stops.