Due to the angle of the photo one cannot see the thickness remaining. Second is a heli-coil doesn’t remove much more than the thread depth when drilling. Third heli-coils are stronger than original because it is slightly larger and the coil is applying side pressure due to the angle. It is also smoother and usually better material than the original. Volkswagen had stainless thread inserts in their cylinder heads same as a heli-coil from the factory back in the 50s or maybe even further. Threads that are frequently disassembled especially in soft metal are better to have inserts. Some vehicles like Mercedes supply a guide stud in the tool kit, I have a couple in my toolbox. Might not be a bad Idea to make one. I have made several for installing transmissions and differentials