So, I went back to the bathroom with a folding rule. I find 80 cm width more than sufficient. 110 cm depth is admittedly rather tight, but it actually works with the space-saving toilet from WICI Conzept. Having the guest toilet in the stairwell due to a house separation is certainly not ideal. We'll see, maybe we'll move it in 25 years.
I designed the two office rooms according to our requirements. I just need a small room where I can work and make phone calls undisturbed. My wife wants a separate room that she can lock to leave things "out" sometimes without the kids getting to them.
I'm not saying the plan suits everyone, but we think it's really good. We're still considering the kitchen island in a different place. My wife doesn't really like the idea, but I still wanted to draw it again later.
I don't really expect peace in the dining, living, or kitchen areas. Four kids always bring life into the house no matter where, and we love that.