Floor plan of a small bungalow with a wooden facade

The problem or one problem is that you are planning something in which you really want to live in x years. In between it is something else, e.g. a holiday home.
Now you yourself are apparently still quite far from "old age" and therefore do not know your interests, needs etc. We, for example, built a house here for the two of us; if it had happened 20 years earlier, it surely would no longer fit our lifestyle and circumstances now. I have changed, as have my feelings regarding privacy, noise, need for quiet, general togetherness, and much more. How could I have foreseen that 10 or 20 years ago? I rather consider building a house for the distant future not a good idea, and if so, I would plan it without children, who then would no longer be children or would be adults, and if you still wanted to live under one roof then, you should plan it completely differently, e.g. with separate entrances etc.
When you ask here for a "good" floor plan, that surprises me, because no one can tell you what will be "good" for you in 10-20 years.
Of course, you can build any house that might perhaps fit more or less, but I would never build like that.
You need privacy (maybe at some point) even between spouses, especially with adult children.

If you write here that the wood species should be larch, that is quite nice but especially shows that you are putting the second step before the first, and one often stumbles doing that; also, you chose a house type and now try to squeeze some rooms with unclear usage profiles in there... I fear that will not go well.
In what year do you want to live there as your primary residence respectively, and how old will you and the children be then?
Why don’t you build a house for yourselves as a married couple, where you can be as a family until then; for example, children can also stay overnight in one room, so that would avoid the search for the jack-of-all-trades solution.
As already says, you should answer all questions AND enter furniture, windows, doors, etc. all with real dimensions.

[*]Building later means having no house until then and thus being able to use the existing plot worse. It certainly won’t get cheaper by then either.
[*]There is a need to submit a building application by the end of June. Therefore, we do not have to build, but it would be a pity to have only done the planning for the authorities.
[*]Doesn’t it apply to almost every construction project by people under 40 that you have to make compromises in the long term? No house adapts to use, and you should never build. I hardly know anyone who builds anew at old age. Everyone stays in their houses.
[*]The worst case, such as selling the house or having to move in early out of necessity, can never be completely ruled out. Therefore, so far necessarily rather 4 rooms. I agree that means compromises for planning.
[*]It could become the first residence if the children take over our apartment. We would then be 50 years old and the children 20.

Regarding the then older children/young adults and equally between the spouses. Health issues in old age are not uncommon, and it may well be that you also maintain privacy among each other. At acquaintances, he hears poorly and always has his radio/TV loud or therefore talks loudly on the phone. She is hugely disturbed by that or the topic of snoring, being bedridden, etc.

As long as you build a nice house with a well-thought-out floor plan, I see no problem there. I would never build a house differently against my needs just because I might be able to sell it better later. Furthermore, it is so that you might then address a larger target group with it, but I do not necessarily believe it will sell better (or for more). Then you just won’t have 50 interested parties, but maybe three. My current house would also not necessarily be suitable for a family or would have to be adjusted somewhat, but it should be nice for us now, and I am sure that I could also sell it well if necessary. Such statements are always too superficial for me and would not influence my planning.
Show your original idea with 3 rooms and answer the questions here so that one can better understand your circumstances.

The floor plan is very interesting. Is it dimensioned somewhere?

Basically, there is still no place for the fireplace. That has caused me the most problems with small floor plans.