One could also argue more simply that only service phase 2 was performed; a design and an initial cost estimate. Since service phase 1 is missing, I find the view that all service phases were commissioned shaky.
Good argument. Although this would be very unusual, it is understandable, and thus only the remuneration for service phase 2 would be owed (and termination without further ado would be necessary to prevent anything else from proceeding afterwards).
I don't know. I'm just often annoyed by discussions that run without any basis or actual value. It's basically a discussion with few solution approaches. Always this could, would, etc. You want to help, but the reason for the discussion is missing. Much has been said on many pages. At some point, you also have to take action. would also do it. Some here can do it. Just having built a house often isn't enough. Those who deal a lot with house construction, can read designs, take 10 or 30 minutes, and then find honest words. Personally, I quite often see things that can be cut without destroying the individual added value. But even that sometimes has to be used.
Oh dear dear OP - this sounds very familiar to me. Sounds to me like you fell for the architect who never wanted to do more for you and is now speculating on a cheekily high settlement amount, which of course will not be reasonable. And if I am right and there really is intent behind it, then you will pay it too... GD
Even if the architect may not have acted very professionally, one does not have to assume malice on his part. Perhaps a friendly inquiry will reveal that, from his perspective, he delivered service phases 1-2 and derives a fair price from that. The OP seems to fear that tens of thousands of euros will be demanded now - without even asking whether that is really the case.