Nintendo's pre-production of the game, according to Nintendo was that 2D version of BoTW that they created. That was done in 2012 and development started on botw in 2013. It was showcased 17 months later in it's first trailer, but the lighting engine was done for wind waker hd and other elements of the engine and game were borrowed from previous works. It's all in the GDC developers talk that he did in 2017.
It was roughly 5 years development in total (6 if you count the DLC production), with roughly 4 years of actual production.
If they started development in 2018, I wouldn’t expect a new 3D Zelda to be finished until around 2023. That’s too long of a wait; therefore there is a place for a smaller scale Zelda game for Switch in the interim, along the lines of ALBW.