This is absolutely true, and actually the mutations are usually quite diverse already, so it's thought that multiple neoantigens should be targeted simultaneous

This is absolutely true, and actually the mutations are usually quite diverse already, so it's thought that multiple neoantigens should be targeted simultaneously. However, no matter how much you target, the diversity and evolution of cancer always wins. Therefore, the goal is not to target every last cancer cell, but to trigger enough of an immune response across a wide enough variety of mutations for the B cells to adapt to whatever else they find. This is also the theory of checkpoint blockades, and they will probably be used together.