I recall hearing about that would be interested to learn more.
AFAIK, Gene's only written reactions to ST VI were based on the shooting script. Before they started filming. He wrote a long, angry memo. And Nick Meyer chose to ignore most of his input.
Months later, his chauffeur and carer, Ernie Over, took Gene to see the roughcut of ST VI. He was in a wheelchair, dibilitated from an earlier series of strokes. As Ernie told me
in person in December 1991, that Gene watched the movie then said, "I am sure the fans will love it."
Not long after watching it, GR took his last trip to the hospital. It didn't sound to me like he was in any condition to race-home-and-a-write-a-long-letter, as was the rumour floated around later.
Well for sure someone else did the physical writing but the story is that he wanted them to cut out huge chunks of the film because he thought starfleet couldn't have people who would behave as some did in the movie.
Was that not the memo about the shooting script? GR also asked Meyer to reconsider using Saavik as a conspirator, because she was "beloved by the fans" - which angered Meyer, since Meyer had created Saavik and GR had originally not wanted that character in ST II.
Of course, Alley refused to return anyway, so Valeris (Kim Cattrell) was created to replace her. But it was Saavik in the shooting script.