Funerals can be therapeutic, but they can also be traumatic. Not just because of traumatic deaths, but because of circumstances like you describe where rather than offering a time to heal and grieve with others it would actually be cementing loss and adding to it for the reasons you outlined.
My general approach with tough decisions is to picture myself further ahead in time after I've made the decision. Picturing all the pieces and then seeing what my emotions and reactions are to that. Sometimes big decisions can swamp you, and imagining the immediate aftermath and a few years ahead can help you to see clearly again which path is right for you.
Your reasons for not going are sound, and although when I started reading your op my initial feeling was "oh no, I'm worried you'll regret not going" by the time I'd read through to the end I did not think that anymore. I think your judgement is good.
My view may be swayed here, because the only funerals I've attended have been traumatic (parents). My personal view is that "it's disrespectful" is not a good reason to put yourself through something damaging when funerals are about comforting the people left behind - I.e. you.
I've chosen not to attend other funerals, or been unable to attend them. Some of close family, some of people I admired and respected and was deeply saddened to lose.
The most recent of those I worried others would think I was being disrespectful, but people were understanding. I've never regretted not going. I still grieved for and remembered that person. I paid my respects. On reflection, I didn't believe they would have wanted me to attend their funeral if it was going to cause me harm instead of comfort. I wanted to remember their last presence in my life in a positive way, not the bleak funeral scene. I really struggled after the funerals I did attend with feeling it had tainted my memories of them in life.
The last visit with your dad sounds hugely important and special to you. Do you think that is a final memory that would be comforting to you as you go through life? Or would you rather have your last memory be his funeral? Or do you think you'd be able to remember them separately?
My gut feeling is you won't regret not attending the funeral for the reasons you have, and in the circumstances described, especially given your final visit. But if you're not sure, could you leave yourself the option by travelling there and listening to your instincts once the time approaches? Maybe even being nearby as it's happening, without attending, could be a comfort. It really depends on you.
I'm wondering if some of your "what if I regret it" worries are coming from being so far away and knowing you won't be able to change your mind on the day (and therefore making you feel powerless) rather than because you will regret it.
Having a memorial with your family sounds like a beautiful idea. If you did go to the funeral and found it exacerbated some of the loss, holding a family memorial afterwards might ease that again and help you all to carry both his loss and his memory as time goes on.
I hope you find a way forward you can have peace with and I'm really sorry for your loss. 