Estimate of construction costs for a single-family house in the Tübingen area

And what stops you from clarifying exactly this BEFOREHAND, in order to plan it accordingly into your construction?? Friends don't have to help with building a house, who even says that or assumes it? I don't.

Due to the distance to family and friends, I also had little support with self-performed work. I never assumed otherwise either.

This "clarifying beforehand" can easily be a year or more between the agreement with the buddy before signing the contract and the actual self-performed work. Especially since the date for the self-performed work cannot be predicted beforehand. In the meantime, a lot can change in the lives of the helpers. Suddenly a vacation intervenes, a new partner, offspring, a new job, etc. I don't want to say that it can't work, and that maybe another buddy can step in. But not everyone has a huge circle of friends where three others can be on standby.

I have been following the forum for over 12 years and have often read that a lot was still done alone because the helpers didn't have time every weekend after all.

I think no one has to (or will) convince the other of their point of view here.

Both experiences exist and are legitimate – which one is more common in reality cannot be resolved here. For the thread creator and all other interested readers, it was certainly helpful that two very different viewpoints were presented with conviction and in detail. Based on this, one must then look at their own case and assess their own circle of friends/buddies. If one is honest with oneself and the situation and comes to the conclusion "yes, I can do that with my buddies," then one does it that way – however, in the case of failure, one cannot later say "I wouldn’t have thought that before," one has to live with that.

This "clarify beforehand" can sometimes be a year or more between the agreement with the friend before signing the contract and the actual own contribution.

That's true. A long lead time is potentially difficult. Therefore, some judgement and flexibility as well as clear communication are required in the planning.
Judgement: If you define concrete work packages that can be done in a day, an evening or a weekend, a more concrete commitment is also possible. It helps the helper to know what they are getting into.
Flexibility: This concerns the schedule and the order of the work. If one appointment doesn't work out, find another.
Communication: The better the helpers know the construction progress and the associated possible deployment period, the fewer surprises there will be – both for the helpers and for oneself. It makes sense to share something about the context of the order of work so that not only a "what", but also a "why" and a "what does this mean" is answered.
In addition to the environment, project management skills always come into play if it is to work. People also create different "realities" that they encounter in this regard.

So regarding helpers, I am also familiar with all kinds of situations. For example, our neighbors who are building their house entirely by themselves, only the plastering is outsourced. But during the bricklaying, there are also 10 family members on the construction site who take their annual vacation for it. In the years before, they had already done the same for other construction sites / family members. And that's how it goes around in turn. It saves a lot of money but you also put in an extreme amount of time. On all construction sites, not just your own. I think you know yourself how much you have helped others and thus you already have a good reference point for how much help you can expect.

is, but not in the decision-making process

One question, how do I best find a freelance architect who charges a flat fee in comparison and/or what does he/she specialize in, e.g., rules, standards, etc. in the specific building area? I only know the portal a better. Based on research on YouTube, for example, or by "googling." I would need help with the aforementioned decision-making process here.

One question, how do I best find an available architect who charges a flat rate

Call and ask.

What he/she specializes in, e.g. rules, standards, etc. in the specific building area?

He does not need to know a specific area. That's what he studied for. You can ask what kind of houses he specializes in and have him show you (references).