One weird thing we have is that part of the interest you pay is tax deductible.

This matches the USA system for mortgages.

for this reason there is a type of mortage where you first only pay the interest, and slowly start paying off more and more of the mortage, which means your net mortage fee slowly increases over time, which is nice if you expect your income to increase over those decades.

This sounds new to me. In the USA we do have amortized mortgages so a very high percentage of the monthly payment is interest with little going to principal. Over time that relationship flips where you’re paying more principal that interest. However, in our system the mortgage payment stays the same, only how much of that fixed payment goes to interest vs principal changes.