Well, funerals are for the living, not the dead. Humans aren't the only animals who have such rituals. Funerals can provide comfort for some, supporting one another in grieving and acceptance, while for others, it can bring out the worst in them and they can inflict that worst on others. It's messy and imperfect, just like life. I've often said that death brings out the idiots and the assholes, and I've seen it happen many times.
Personally, I don't want my parents to claim my body, because they rejected me in life, so why claim my body when I'm no longer in it? I also don't want them to have a funeral and receive support when they gave up that right. But they're assholes, and they'll do what they want regardless of what is rational or just, let alone what I request, that is their consistent way; it's beyond my control or influence, or they would have honored my boundaries and autonomy and changed when I was a child, a teen, or an adult. If I were to leave them a note, which would be a waste since they have filters that alter anything I say, no matter how clearly, I would tell them that I will neither haunt them nor try to comfort or guide them, as I am done with them and with life, and whether they suffer or experience well-being, that is their responsibility and I will not interfere either way. They still would misinterpret it, or see what they want to see. I give up the battle, because I will never win. Instead I focus on what I can control, a peaceful death at the time that is right for me, according to my own situation and not according to their convenience or wishes, since my own is of no concern to them.
If you have to make threats to get anyone in your family to do what you want them to do, I wonder if that's because they already have a history of not honoring and respecting your wishes. Unless they are superstitious, I doubt your desires will override their desire to meet their own wishes and convenience, even in the face of coercion and threats. If you have not been able to protect those you want to protect while you were alive, that's probably not going to change, either. Maybe all you can do is state your request clearly and adamantly, and hope that they will honor it. But if they tend to function in idiot or asshole mode, then that is probably what will rise up to meet and deal with your death, and to deal with others in relation to your death.
I don't say this to discourage you or to engender feelings of impotence, but to perhaps bring up consideration of what is within your control and what will bring reason and rational acceptance, if not serenity. Of course, these are just my opinions. If anything I said doesn't serve, I accept and respect that.
Sending compassion for your struggles, worries and frustrations. I honor whatever you decide to do. This stuff isn't easy.