Not sure how different or similar it is to the US. In the US you could have a power of attorney for someone other than your next of kin to carry out all health care and post-death wishes, basically take the place of them as next of kin, make them the executor of your estate, write a will with your wishes, and pay in advance for the cremation, though that last part can cause suspicion if you're young and in good health.
My parents I'm sure wouldn't follow my wishes either, so I decided to not say anything at all and let them do what tf they're gonna do since they will anyway. But we've been estranged for years, I know it's not the same for you.
I hope you get some helpful answers here, or can find peace in acceptance of how they are. At least if you write a note you know you tried, and you can write notes to others in the extended family to try to pressure them and make them feel like shit if they don't comply. Death and funerals tend to bring out the assholes, so if someone in the family has a grudge against your parents, I'm sure they'll love to stir things up and make it very uncomfortable for them.