One thing you could do ( or get the mechanic to do) is that before the new subframe is fitted, get it sprayed inside ( especially inside) and out with a good protective coating. There are several good coating available for this if you google it. I remember once upon a time, there was a long flexible probe, with a type of head on it, that resembled the head on a drain jetter, and it sprayed literally in every direction.....if you drilled a small hole in a 25 ltr container, and inserted it, every square inch internally would be covered. At the time, we used a protective wax. It was liquid in form , but hardened enough to form a thick protective coating.