Yeah it's a bit silly, but the best way to play around the system is to research all the district technologies first. Then, while you're still low on Technology/Civics, you place down as much districts in your cities as possible, without actually building them right away. That way you get cheap districts. This is another thing that makes Germany so OP, because you can place more districts, while the cities are still very low pop.
At the same time, it can be very punishing to advance through Technology/Civics trees too fast in the beginning. If you get too far ahead, without placing/building districts, your commercial hubs and harbors suddenly cost a fortune. That makes it better to hold of on science districts in the beginning and get your commercial/industrial/harbors up first.