And, make a plan that includes old and new (and always keep all plans in one orientation and do not change the orientation with every post) and label what you imagine where.
To build on the gable side was also the solution I preferred. However, how such a harmless third gable can disrupt a roof structure is often underestimated - both structurally and in terms of cost.
Additionally, we would like to install a captain's gable. It should create more brightness and space on the ground floor
I would leave that out on the upper floor. Adding a conservatory on the ground floor is still affordable. But retrofitting a captain's gable on the upper floor is like once a completely new roof with creamplease. How much did you say you wanted to spend on the renovation overall?