I was the middle of three children born within three years. A few months before my older sibling started school (in a Germanic system), my younger sibling died, which was obviously a traumatic experience for the whole family. It was probably due to these circumstances that my older sibling struggled at school, despite being extremely intelligent. And so when the decision came as to when I was to start (which was largerly the parents' choice), I was 'kept back' a year so as not to be in the subsequent year to my sibling. I started age 7 (and a bit) rather than age 6 (and a bit), so that if my sibling was ever 'relegated' (made to repeat a year), we would not end up in the same year. So the decision was made based on my sibling's (somewhat hypothetical) needs rather than on my own.
Being very academic myself, and much more 'school-shape' than my sibling (sometimes being younger protects you from trauma a bit, just as in your children's case, being younger might have made the transition to Germany easier), I absolutely coasted through school. There were at several points discussions about me skipping a year but I never did. I was just always 'best in year', up to and including at university.
I do think that in such a case you have to watch out for issues around perfectionism - which can be a real life impediment - especially in girls. But on the upside, there were many benefits to being essentially 'a year below' despite being a high achiever. I got to pick all my selective schools/universities, with barely any effort. Leaving me with the time for an international 'career' in my chosen 'sport' (chess) on the side. At age 18, I would miss three entire weeks of school for a competition, return and do well on all tests. That (chess) is also what taught me how to work/study/revise, and that there is always someone out there who is 'better' than you.
My sibling and I were always fiercly proud and protective of each other. And despite my massively better academic record ('straight-A-student' vs 'drop-out'), I happily acknowledge my sibling's academic intelligence as probably higher than mine.
So I think it is possible to make it ok to 'keep back' the younger child, even if it is mainly for the sake of the older child.
But recognise that each child has their needs, and they won't just be solved by not accelerating the younger. The older will still be(come) aware of the younger's abilities. The older still needs special attention regarding the transition to Germany, which apparently has been less easy on her. She will still need something to shine in, something that is 'hers', and recognition.
And the younger will need (at some point) to learn how to work at something, to deal with things that don't just come easily instantly, will need to avoid the pitfalls of perfectionism, and will need something where he can experience the immense satisfaction of achieving something 'hard'. You cannot really expect school to provide this - not if he skips a year nor of he stays put.
Completely separately, I have a Swiss friend who's eldest was put forward to skip a year. Ultimately, they decided against it, he got to join a 'G&T' club (one a week, skipping a regular class instead), and he has done fine so far. What I'm saying is that you as a parent have a lot of input here - it is not simply the school deciding.
All that said, if you were to decide on acceleration, then the end of the first year of a two-year Einführungsstufe would be the perfect time. I imagine it would be fairly frictionless, socially and emotionally, for him. The whole point of these Einführungsstufe (they're widespread in Switzerland now) is that children move on to 'proper' school when they are ready, not when they reach a random age number.
Accordingly, there will often be a wide age range in each year group, with some children older than the main cohort, and some younger. (Whereas in England you have a wide range of abilities in each year group, with some children years ahead of the main cohort and others years behind.)
And because it is a normal thing, it is not 'age' that determines school year, this is something that could be more easily discussed with /explained to both children, than if you were in an English system where the vast majority of children are in their age-specific cohort. So I think that too would be possible, and probably most in line with the local 'system'. I do tend to think it is often more pragmatic to go with the system and make up for the downsides, than to go against the system and always be fighting on all fronts. E.g. if you decide against acceleration, don't expect the school to massively step up re differentiation or such.