You have to let them, and your relationship with them, however negative it is, go.
It sounds like you hold a lot of guilt, and worry alot about what others think. It's not like you are pushing them off on the iceberg yourself!
You could ask them to send a WhatsApp to the family/ friends group or post publicly on FB that they are doing this for themselves and do not want ANYONE but the authorities to have to pick up the pieces. Rather like a DNR but for you.
Yes they sound selfish, but partly because you are insisting on taking up a rescuer role at some point. This is your guilt, your emotional investment, not theirs. You will be stronger for allowing them dignity in their dying. You work in healthcare so you know what this is.
It sounds like the dynamic may have been/is abusive and or manipulative on their part. Put on your big girl pants and call their bluff.
You might want to get some grief counselling in advance of them going. Many of us have grieved relationships before they have ended and it makes the ending somewhat easier to bear. Note I said somewhat.
Parents can be shits, I know, I left mine behind decades ago. I had to be the adult in the drama triangle. I rarely think of them now.
I'm not religious, but dying is either the end, a relief for me. Or not the end, a relief for those who believe that.
Respect their wishes as grown adults with capacity, take them at their word, let them go, and DO NOT pick up after any mess they leave.
Good luck babes xx