This is a beautiful and moving tribute. My father also passed away very suddenly almost ten years ago, and like you I still have his last email in my inbox. He died of a sudden heart attack doing what he loved: climbing a mountain. Although it was a shock, in retrospect I can’t think of a better way for him to have gone as he would have hated getting elderly, infirm, etc.
I should add that I’m also a Christian, and I believe God showed me in a weird waking dream/vision what actually happened to him. He was taking a rest near the summit. Then when he got up to continue, he found Jesus at the top, and realised he’d left his body behind. I know some people will probably laugh at me reading this, but after what I read in your piece I thought it important to affirm what you said about the dream you experienced. God is a great comforter, and I believe you will see him again.