Well, if that didn't come across - I actually didn't feel that Kirk's was a tragic ending. I always did imagine Kirk dying in the line of duty, and so he did, fighting for something worthwhile. Certainly not of old age in retirement. And against all odds, as he went he was with one of the few people in the universe who could understand him because they too sat on the same chair as he did. The specifics ("freak bridge accident") don't matter; all death is ungraceful in its mechanics.
Re Kirk behaving manically in the Nexus, again, worked for me. The character did have a manic streak at times, and he certainly also consistently showed an unhappyness with aspects of his life and life choices, as well as changes in his carreer and role in the universe. To me the events in the Nexus nicely played up themes from TMP and Wrath again, and bookended them to a degree. Kirk was not perfect, it's what made him interesting, and he often ultimately derived strength from his inner turmoil, as here.