Offer architect house 150sqm / 750 thousand €

....I had calculated with 3000€/sqm plus architect

So even 3000€ I find quite decent.

750k.. really disproportionate. You get 150sqm with basement, pool, outdoor facilities, plus cleaning lady and gardener for at least 20 years :p

The fee is okay so far. At least based on the amount mentioned. What amount did you assume?

Single-family house fee zone 3 with minimum rate is, in my opinion, appropriate, maximum quarter rate. If the architect already estimates a single-family house in zone 4, he is either someone who can charge such rates (whether that is necessary?), or there has been a major misunderstanding.
With the word Bauhaus, a significantly higher construction sum is assumed, and the architect's own fee increases accordingly, especially in zone 4.

...Bauhaus style. Cubic cube with recessed double garage and flat roof. Interior finishing of high standard - mind you, NO luxury. ....

There are some small terms in there that tell the architect he can make a killing :)
You can then gladly ignore "no luxury."
"Normal standard" and "modern construction with flat roof" should be enough for the next architect in the future ;)

That confirms my impression that unfortunately most architects are worthless! How many cubic meters of volume were assumed? We were told back then that you have to calculate around €420-450 per m³ on average for a standard fit-out, plus architect's fees and land costs. That sounded plausible.

We also had some conversations back then... most are so arrogant and stuck up... unbelievable! Others, on the other hand, are eco-fascists and want to sell you a KFW40 house at all costs. I recommend you look for an architect who has a reliable network of craftsmen he regularly works with. He has them better under control. Both in terms of scheduling and quality. And for you, that means more planning reliability, faster completion... and a budget overrun of maybe "only" 10% and not 20% ;)

Others, on the other hand, are eco-fascists and want to sell you a KFW40 house, come hell or high water.

That's no longer the devil, that's already Oettinger: when more petrochemicals are used in a house than anything else, that's rather the opposite of eco-friendly.