Nice to read that other lawyers with even better incomes also struggle with such a decision. Although at least some jurists can calculate contrary to popular belief, it remains largely an emotional matter, as has already been written. I therefore understand the desire for some exchange here in the forum. By the way, our decision was also an emotional one – the monthly burden is high for us due to a lack of equity (both academics with long studies, no wealthy parents) and I see a clear advantage with you that if you spare equity, you have a huge buffer for any uncertainties. We lack that, which already causes some grim moments before construction even begins.
So what I mean is: Use the low interest rate environment, maybe put a fraction of the equity into the financing to get below the 90% loan-to-value and possibly reduce the interest rate even further, and enjoy what we chose the high burden for: your own piece of green space (also coming from Bavaria the plots around Frankfurt are tiny), the feeling of freedom for your own decisions and the hope for an increase in value that should enable a later renewed construction with changed requirements (less space needed inside when the children have moved out, more space needed outside when retirement is spent at the flower bed).