I hate to say this, but I’m not sure losing a loved one to dementia isn’t just as traumatic, and I think the trauma could be mitigated here by communicating with the people they love.
When/if I am diagnosed with a terminal disease I hope I will have a conversation with my wife, the rest of my family, and my closest friends about how I want to die and what a life worth living looks like to me. I like to think that I’ll leave this world by my own hand, hopefully surrounded by the people I love, or at least with their understanding and support. I don’t think that’s selfish.
OP, I think you have time to figure out what’s best for you and yours. We’re all terminal in the end. I wish you a slow progression, a meaningful life, and a dignified end someday.
There are plenty of very happy old people. I’ve watched my parents flourish as they age. My great uncle always talked about making the last year’s of your life the best years of your life. I don’t know if they were for him, but he was still dancing until the cancer took him last year at 94. Yeah we all fall apart and die, but you’re best days can easily be in your 70s or later.