I am sorry for your loss. I really think that more states should have the Death with Dignity laws or whatever they are called. Then people could talk about these choices and include their loved ones and be with them when it happens, rather than planning it in secret and leaving their families to wonder "what if" . I also believe that people think of suicide in a certain way, like it has a stigma attached to it, where people think there was something "wrong" and maybe it could have been fixed, which leaves families feeling guilty. And this particular circumstance is not one of those where anyone should feel guilty.
I say that for this reason. My dad died of cancer. He'd had several surgeries over the course of 6 years, starting when I was in 3rd grade. When it came back the last time, he decided he'd had enough. He'd done the surgeries, done radiation, all the things, and his quality of life was not as it should be. His cancer was in his mouth from chewing and smoking (this was early 90's and he was born in 39, so common for that age group). So because of that, the last surgery was pretty extensive, resulting in him needing a stomach tube to eat, and a trach. He was ready to be done. So he did Hospice and all the things and wanted to die at home, and had a plan. However one morning he woke up and didnt know who any of us were or why he had a stomach tube etc, and had to be taken to the hospital and put in a come until he passed. My mom wanted to follow his plan but was afraid she'd go to jail, and I was only 14 so she didn't. If those laws existed then, he could have passed how he wanted to.
The death of a parent is hard, no matter the circumstance or the method. My personal feelings is that your father had made a choice and went quickly. And even tho that choice might not be one you'd have chosen, it was one that ended the suffering quickly, rather than a drawn out illness. I think focusing on that part, rather than the method, may make it easier to cope with. I really am sorry for your loss, and I hope that once the initial grief wears off that you can find some solace.