I am asking for help with my father’s treatment (prayers, advice, financial assistance).

Hang in there. When my mother was diagnosed with stage 4 stomach cancer, I cried and wailed before going to see her. Then, I arrived "prepared": the verdict had been delivered, and in our case, we could only work to delay it for as long as possible (in the end, we fell into a rare statistical range, exceeding the average lifespan). We were lucky with the oncologist who administered the chemotherapy, but it took an unacceptably long time to get to him, and at every stage, the Russian "healthcare system" consumed time.
In the final stage (when the chemotherapy stopped working), I distanced myself as much as possible and treated taking care of my mother as primarily an important and responsible task (this helped me stay focused and forced my brain to work on finding solutions to the problems that arose). It is very important that all relatives and loved ones have a shared understanding of the situation. Unfortunately, we didn't achieve a shared understanding: after my mother, my father also clung to the illusion that she would recover, and my sister got stuck in the bargaining stage, hoping for a longer life. So, only I reached the acceptance stage, and I was the one handling all the medical aspects. However, we all failed to cope with the psychological aspect in the end – we couldn't shift to a positive mindset. And this was strange to me because my mother was involved in "spiritual development" and had previously been calm about death. But we had another small, yet significant, issue. Until the very end, she allowed her Guru, Ivanych, who was the leader of the meditation group, to "feed her lies" (he wasn't taking money, but he was definitely manipulating her and the others). When I was around, I simply wouldn't connect them when he called again, but the others would.
I'm expressing everything a bit chaotically (it's difficult even after a year to delve back into that period), so I'll try to summarize.
All relatives should understand that if stage 4, inoperable cancer is diagnosed, it is a death sentence. All we can do in this case is to maximize the delay of the sentence and create a favorable atmosphere so that the person can complete their earthly journey with dignity. Ideally, this should be a team effort – someone focuses on the medical aspect, someone on the religious aspect (if the father is religious), someone holds their hand and talks about everything they want to talk about. And together, create an atmosphere of love and care, keeping away negative, irritating, and energy-draining influences. If it's more comfortable for the father to believe in illusions, let him, if it really gives him strength. But relatives don't have that right; otherwise, we lose effectiveness and start acting like the swan, the crab, and the pike.
Determine the father's level of awareness based on what you know about him (personally, I would prefer to know – because how can you guide him to the "acceptance" stage if you don't know what's going on with him?).
When he's around, everyone keeps their composure; if emotions overwhelm them, they quickly apologize, say they have an urgent matter, and leave to cry so that he doesn't see or hear. They wash their faces, compose themselves, and return to our palliative work, with a focus on maximum sensitivity and openness to the person (I was never as close to my mother as I was in her last days). And may God help us! We are all mortal, and we will all be there.