My advice would be 1) to not phrase decisions as suggestions, 2) motivate/justify with hard facts ("an outage occurred, we are having a meeting to discuss causes/mitigations"), 3) if post-mortems are perceived as publicly assigning blame, it can't work and the culture is wrong, and 4) never motivate decisions by identity ("my background is fancier than yours so get in line") which might be happening implicitly if decisions aren't sufficiently grounded in details and facts.
I was about to write the same thing. TS, if you feel strongly about the need for a post-mortem analysis then it is: "We WILL do a post-mortem analysis, and you WILL analyze root cause issues and you WILL help me write a report about the incident. It has the highest priority." It is not: "We should do a post-mortem analysis, you guys agree it is a good idea?" TS comes of as wanting to lead by persuasion, which imo very often doesn't work.