My sincere sympathies too Faraway Gran. I wish I could offer help or useful advice, but you have clearly given this much thought.
It doesn’t have to be an “either/or” though does it? (Extended pain/Dignitas) Macmillan Nurses or other Cancer nurses, Palliative Care nurses or consultants too should be able to perhaps allay your fears and reassure you.
Our local hospice offers counselling, “outpatients” appointments, pain management and support for families amongst other things - or at least they did before Covid.
I wonder if a conversation with someone from a hospice would give you reassurance and down to earth guidance.
Dignitas are expensive as I understand and I also believe you would need someone to accompany you.
It is a brave decision for those who take it, but whether nor not it is the right decision is down to each individual.
Do please try to talk this over dispassionately with someone who knows what they are talking about and whom you trust.