Is membership in the Association of Private Builders worthwhile?

Whereby that also has its limits. Certainly, the site manager (these are almost always civil engineers, so I find it somewhat disrespectful to put this in quotation marks across the board, as if they were unskilled workers with a 2-week training in construction) is paid by the main contractor and therefore biased.

I always put the contractors' "site managers" in quotation marks, but never with the background of doubting their competence (which unfortunately only benefits the client in a very limited way). Nor do I ever claim that architectural site management would be carried out by better-trained persons. A private house is best planned with a small architectural firm with only one professional (architect, often university-educated, so not from a university of applied sciences with relevant practical experience as a qualification route), who shares site management with his assistant (typical qualification: drafter) – in this case the site manager is not even a civil engineer or foreman. But here the site management has undivided loyalty primarily to the client party, and such a small architectural firm has projects in the local area that can be counted on one hand. Occasionally, as suspenders to the belt, I also recommend involving the construction-accompanying expert here. They were often themselves formerly contractors' "site managers," and have left due to burnout risk to become self-employed. Some even temporarily return to permanent employment during the family formation phase.

They only ever call me to systems that are malfunctioning and not to those running perfectly, so that I may admire the faultlessness of our work...

Exactly there lies the problem, which also often leads to burnout: lots of "firefighting" / few successes, staffing ratios dictated by cost-cutting (the site manager is supposed to be cheaper than the even higher catastrophe costs without him), accordingly lots of kilometers traveled. In a way constantly like a self-driving emergency doctor on the bike, only without blue lights and rescue lanes. The question of bypasses is only "when." Because competence is ambivalent: professionally, it may facilitate everyday life, but on the other hand, without it one would also have fewer conflicts with management. I prefer to roll my calm freelancer ball as a universal know-it-all (unfortunately here without punctuation emoticons).