Hi Melissa. Thanks for sharing your story. My dad also had dementia, though different from your dad’s. It wasn’t quite the rollercoaster you described, but more like a plateau. His cognition would drop, level off for a matter of months and then drop again. He was belligerent as well.
To answer your question, I do believe that when we pass on to spirit we are at peace and that your father is as well.
My condolences on your loss.