Hello.
I Will report my opinion based on my knowledge.
Sexual illness: bad idea. You'll have a long way to go until you die. Denying treatment may shorten the journey, but it will significantly increase your suffering.
Ebola: Bad idea. Highly fatal. It is different from covid, because the virus is in the transmissible period when the patient is isolated with all the symptoms. So it is necessary that you live in an endemic region and have the "lucky" to have an outbreak. Otherwise, you don't have access to the virus or who is infected. And there is also a lot of suffering (bleeding for days...)
Cancer: bad idea. Cancer usually gives people opportunities to say goodbye, however there is a lot of suffering, for those who die and for the people who love us. It's a terrible disease. There is a lot of suffering and you have chances of healing. The duration of suffering depends on several factors (type of cancer, location in the body, immune system, access to treatment and medicines). In the end, you may or may not be cured. When not, in the end there will be so much pain that no medicine can help you. The body slowly collapses, even rejecting food. There is so much suffering here that even those who love you may wish you to die soon. If you are going to die of cancer, the "ideal" is to have a highly aggressive cancer that kills in a few days (leukemia, pancreas, brain are usually more likely to kill in a few days, but it's not the majority and there are no guarantees.)
There are many highly fatal diseases that kill quickly, in days or even hours, however there is a lot of suffering involved and access is not simple, such as rabies disease, meningitis or sepsis.
As I said in cancer, the time to death will depend on several factors, such as access to treatment, the immune system, type of disease, location in the body, etc.
Denying treatment is a right in my country (Brazil) as long as you are conscious and have mental health. However, if you have life insurance and deny access to treatment, it could be considered suicide and any benefits will be denied. Think about it before rejecting treatment
I believe that there are other, less painful and obscure ways to seek this peace. But unfortunately it is denied to us and many end up choosing alternatives that cause more suffering.