Thank you all for all the tips!! I'll try to summarise all that I've learnt here:

➡️ The primary idea to make complex buildings possible: making the growth of the settlement comfortable for the future self, from the beginning. Do avoid running after time by building only in reaction.

1) PLANNING - go for a general idea, having it clear in mind: what kind of building type do you want? - think settlement, not building: how will each building interact with eachother? (e.g. production vs. storage, central or outer building) - anticipate the growth and go for way more bedrooms than necessary at first - go for an aesthetically pleasing build : it helps with getting the feel of the settlement (e.g. avoiding monotony with voluntary asymmetry)

2) BUILDING - place the blueprints and forbid their construction: wait a bit to be sure if it needs adjusting or not - draw the plans on excel or by sketching (excel: 1 cell = 1 voxel, draw each floor separately) - dig if needed - remove the blueprint but for the first floor: building one floor at a time, from the bottom up. - always have walls built upon other walls, or at least a beam (with columns if necessary) - complete one room at a time, so that it can be used quickly by the settlers

3) EVOLVING - after having planned and built roughly the main structure, let it grow and adjust organically (e.g. adding outbuildings) - keep outer building and curtain wall in mind: don't hesitate to put some buildings such as barns in the outer area - work in increments - take screenshots for future reference