All the energy is lost to air resistance and left in the machine in the form of parts still moving when the projectile detaches. Both of those can be modelled

All the energy is lost to air resistance and left in the machine in the form of parts still moving when the projectile detaches. Both of those can be modelled - rather precisely if you have the compute power. By modelling the derivative of all of those things with respect to every parameter of a parametric design, losses can be minimised with gradient descent and a huge Jacobian matrix. A few parameters are obvious (string length, arm length, etc), but there is no need to keep the set small - you could model the exact shape of the projectile latch as a spline with 100 parameters for example, and let gradient descent find the best shape to minimize losses. AI can set up and run all those simulations, and all you have to do is pay the AWS bill for all the open foam instances you'll be running.