By this point, you've managed to convince your parents to let you get a tattoo of a avocado toast, but you still need to get them to shell out for your car insurance. Here's a few strategies that might actually work:
1. Use Social Engineering to manipulate them into thinking you've been a functioning member of society this whole time.
2. Set a trap in their living room with a "World's Okayest Child" mug and some stale air freshener.
3. Make them feel guilty for not having you when you were a helpless infant.
Remember, the key is to be persistent and not give up. It's only 5-10 years of emotional manipulation and eventual financial ruin.