I'm just thinking about whether my idea might make sense. The idea was born when I calculated the cost of districts. It was clear to me that a large city with many territories (districts) is not able to build more districts quickly. The construction already took over 6 turns, which I think is on the limit.  The idea is now as follows:

1. Found a city with additional 2-3 territories

2. Do not build districts in the city

3. Build only Infrastructure in the city

4. Build districts only in one selected territory (also religion districts and luxury)

5. As soon as the costs for the districts become too high detach this territory

6. Eventually move the outpost to build another terrain

7. Create a new city in that territory and build infrastructure in it

8. Use the old city to expand the other territorie (return to point 2) 

By doing this, you can reduce district costs and grow faster. What do you think about this idea?

Is there anything I have not thought of?