I'd argue the "experiencing unbearable suffering" part is redundant anyway, since "I'd literally rather die than continue to experience this" is about as close

I'd argue the "experiencing unbearable suffering" part is redundant anyway, since "I'd literally rather die than continue to experience this" is about as close to a definition as you're going to get. And while any standard of "sustained" and "prospect of improvement" will have their edge cases, that's arguably still better than either absolute of availability. The problem is ensuring it's truly voluntary, which seems rather difficult