Defining the financial framework, recommendations on house sale / land selection / new construction

I would rather leave it. For the stress and costs of construction, you can commute for a long time and pay taxi fares for the daughter if she wants to go to the city. In addition, it is not clear whether she will stay at home at 18 or not, which is impossible to estimate at 14. The phase around 18 will be interesting enough for everyone, so I would not plan the initial phase in the house. But everyone is different, just as a thought.

We can imagine downsizing to a bungalow with 2 bedrooms and no separate office. So something around 100-120 sqm would be fine.

We did that, but 10 years later. The children were gone and the house was too big. If you are not driven out of the house for important reasons, continue living there and save for the bungalow on the side. It can then have a valley view or sea view, there will be enough money. We decided on 125 sqm and 3 rooms.

(...) 10sqm. Check. Then it's 110sqm for the parents. (...)

Yep. Have fun with a teenager who's being crammed from 14, 15, 16sqm down to 10sqm just to build your new dream bungalow. That doesn’t even fix the bus connection to the club, because you can’t invite friends over to pre-drink in a 10sqm rabbit hutch... This will definitely make the last 3 or 4 years until adulthood hell... No offense, but with your views I sometimes doubt that you have kids living with you 24/7. That’s so outdated, the 60s just called, they want their parenting methods back...

I have already experienced a larger number of changes, constructions, moves, etc. From these experiences, the following questions arise for me:

We wish for yet another change

Which specific changes are we talking about here?

getting out of the comfort zone,

What does your current comfort zone include and what do you dislike about this existing comfort?

which we want to implement with our daughter's graduation in summer 2027.

Then she will already be almost an adult and will probably rather move away from her parents, won’t she?

But now to the topic of finances, how do you assess our situation, also regarding our age, could/should we dare the adventure of building once again.

I would by no means see age as a problem; currently, I just do not see the desired goal. Why shouldn’t it be or become a suitable property that you then live in as a married couple and enjoy the then rather calmer life?

My husband is a bit hung up on his age, I less so.

In what way exactly?

Our daughter is also very enthusiastic about the idea of moving closer to the city, especially with regard to secondary schools (university, college or also training opportunities).

At 14 years old, she is an adolescent and therefore not capable of appropriately co-deciding on such a profound undertaking. Have mom and daughter planned that she will continue living with her parents and therefore also work/study in the same city? I consider that a rather unreliable life model, which can burst (fortunately) faster than a can’s bubble.
Would the advantage then be that the then adult daughter, still living at home, has a location 15 minutes closer to the university or workplace, or what specifically would it be?

But somehow the comfort zone and the feeling of the need for security keep interrupting our euphoria.

You should define the repeatedly used term from your perspective. I quickly read the following definition: "Comfort zone describes an individual area of private or social life characterized by relaxation, convenience, well-being, coziness and freedom from risk". Isn’t this actually what one wants to achieve, especially when getting older and therefore weaker? In my opinion, there must be another reason or drive for this; I would be interested in it. It’s a shame your husband does not join the discussion here, I would find that really exciting and especially constructive.
By the way, I find "euphoria" in most cases to be a rather bad advisor.

Many uncertainties, to which I hope for (maybe only strengthening..) answers..

I would always, and do, reflect on changes in life that one does not simply let happen or endure, but rather takes into one’s own hands in time. Insofar, I would have no concerns about your mere change.
However, from what I read from you, I do not see any reason that would improve your (your husband’s and your) life or that refers to a certain, even if rather eccentric, life plan.
Please explain the life plan so that one can understand it. Your daughter’s is not necessary for this, as she has her life ahead of her, so the plan of an "older" married couple who will soon live together without children.
Actually, without further significant reasons, I consider your plan a nonsense idea, basically building a new house in the same place without need or particularly tangible added value.
If it is just about the lack of risk, I would go to the casino once a month in style, and if the coziness is too much, it can also be reduced easily and differently...

Wouldn’t it be an option to already now and in aaall calmness imagine and optimally plan life as a married couple without the daughter or perhaps in part-time or retirement? Maybe it will then be a nice city apartment with all imaginable comfort and no car or the small house somewhere in the countryside.
Of course, you would currently have to include your daughter and to achieve a healthy distance this would have a significant influence on a floor plan. Why should your husband want to live closer to the university or the daughter’s workplace, which will be elsewhere anyway?
In short: I do not see any real added value or justifying life plan, rather a loss of peace, security and comfort.

One has to say... if the man is already acting up at 49... then it won't be any different in 5-10 years and one should just let it be at some point.
"Hangs on his age," that can also be a subtle way of saying "I'm not interested in another new build, change, etc."
The grass always looks greener on the meadow next door than on one's own. But often that is unfortunately misleading as well.

Building a house is always stressful and time-consuming. You are busy with it for at least 1 year.

That's great for people who simply enjoy doing it.

For the stress and costs of building, you can commute for a long time and pay taxi for your daughter when she wants to go to the city.

I already wrote about the "stress." And about the costs... There are few better investments than in your own quality of life. The prerequisite is that you know what you really want, know yourself well, and are sincere with yourself.