Sometimes it requires a lot of thinking to come up with simple solutions.

Usually you need to consider a lot of alternative solutions in order to come up with the simplest solution and that takes time and a lot of thinking and a deep understanding of the requirements.

When I start a new complex project, in the early stages, there are entire days where I'm just thinking about stuff without writing any code. Then as the project moves forward, I spend less and less time thinking about things until I get to a point where it feels like the code pretty much writes itself because the design is aligned with my original goals.

You just have to be very clear about what your big goals are and where you're going to need flexibility from the beginning.