Planning/Preparation for Building a Single-Family House

Hello, we would like to start building our single-family house in the spring. We are building without a construction contractor, going it alone. At the moment, we are still in the land purchase phase. We have the floor plan ready with the architect. We still need to plan the windows; now I was wondering if it makes sense to contact window manufacturers before talking to the structural engineer. And further questions: - Is it sensible to already go to bathroom ceramics companies now to have the bathroom and guest WC planned? - To obtain offers from kitchen manufacturers? - To obtain offers from door manufacturers? How else could I use the time sensibly before the house construction begins? So that I do not have to make all decisions at short notice. Thank you very much in advance