CCWang, suggest you the INCOSE CEP based training to enhance the core concepts. You can use ‘Product Development & Systems Engineering’ by Christopher Olson on Udemy. I have used the same as a supplement for my ISSEP.
Sunil Pilathottathil SAFe Practitioner, TOGAF, ASEP, SABSA-SCF, CISSP-ISSEP, CPISI, CIPT, DCPLA, CC, CCSP, CISSP, CGRC, CISA, CISM
Perpetual Learner..!