I would write a formal email to my manager along the lines of:
As you know, I have decided to quit this company to pursue my personal career goals. I greatly appreciate your ongoing engagement to keep me on board, and it shows me how much the company values my work. Unfortunately, I have made my decision and committed myself elsewhere and I don’t break my commitments. I would appreciate if we could skip further discussion on that matter as I have nothing to add here. I am happy to do everything I can to provide you with a clean handover.
Thank you for your understanding and for the great time working together in the past X years.
If you get further invites to discuss the matter, just refer to that mail. If they insist on discussing that nonetheless, attend, but just repeat. Don’t give any further details and reasons and especially don’t tell anyone where you are going to work!