I went through it with my mum until she passed in 2021. It was 10 years from the first 'inkling' that dementia was setting in until she died. And for the last 3 years she knew nothing and nobody.
At first it was 'little things', easily ignored or laughed off. But by 2013 she progressed to the point where it was no longer safe for her to live at home, even with my DB there as her carer. She wandered away from the house no matter what we did to secure the doors, she called 911 on DB in the middle of the night telling the police he had a 'woman held captive', accused us both of 'drugging her' and stealing from her. It was horrible. But as is normal, at other times she was sweet and loving, and full of wonderful memories of our family. But ask her what she had for lunch and she'd have no idea. At that point we moved her to a memory care facility where she could be looked after properly in a secure and safe environment and her condition could be medically managed.
One of the best pieces of advice given to us was to, as much as possible, enter her 'world'. Never try to convince her that her delusions were, well, delusions. So if she called me by her sister's name, I answered. If she said 'this is a lovely hotel' about my home, I agreed. If she asked for her long-dead mother, I'd simply say she was 'at the shops'. Obviously, dangerous delusions needed to be 'discouraged' and downplayed and we would just 'hmmm' when she accused someone of 'whatever' and distract her with photos or TV.
Someone who has never been through this with a loved one simply cannot understand the pain. Or perhaps misguided humour is their way of deflecting the idea that it could happen to their loved ones, or to them. I was lucky in that my cousin was going through the same thing with her mother (our mums were sisters) so we leaned on each other. Plus my DH and DCs were wonderful and supportive. At this point I suggest you 'pick and choose' with whom you share information, and seek support groups if you cannot find sympathetic friends or family members. Eventually, and sadly, you will get 'used' to your dad's behaviours. Not 'used to' as in it won't bother you, but 'used to' as in an acceptance of each step on this journey.
Former President Reagan, when he was diagnosed, referred to Alzheimer's as 'the long goodbye' and it truly is. My darling Mum died in 2021. But we 'lost' her years before, when she no longer knew us or even really 'knew' herself. Her death (from Covid) was indeed a 'blessed release'.