In my view, I have always separated the types of rear-facing analysis due to the intent of the review. In other words, I do not think that a lessons learned or retrospectives or whatever you want to call it is the same as a postmortem review. While they sort of seem similar, a postmortem is all about finding reasons for the death...or project failure or performance degradation in our case.
Therefore, postmortem or forensic analysis or autopsy is the right word to use.
There is no reason to soften it up. It's what it is. We experienced a crash and we need to find out why it occurred to do the things to attempt to prevent it in the future. When doctors perform autopsies, when the NTSB investigates a crash, when a forensic accountant is sifting through financial documents, they don't soften things up so we can digest them better. We as PMs do not need to do that, either.
However, for a historical review of what was done, trying to learn both from what went well and what didn't, and for reviews that we schedule after all work, call it whatever resonates with your stakeholders.