The only overdose I can think off the top of my head that could make you CTB fast enough for your situation is cyanide poisoning.
...I kinda don't like recommending cyanide poisoning because while thankfully it's very fast acting (it'll only take a few mins to CTB), despite what movies lead you to believe, death by cyanide has been actually been described as looking extremely painful (apparently it's been described before as feeling like if your insides are burning). Also worth mentioning if your parents manage to catch you on the act if they bizarrely happen to have one of its various antidotes on hand, as long as you took a fatal dosage of cyanide, chance of being saved is incredibly unlikely.
Sadly I have no idea how to properly get my hands on cyanide. I gave up going the cyanide poisoning route once I found out how painful death by cyanide poisoning can be. I even briefly checked it up on a dark net market out of curiosity and couldn't find any listings, although I was already expecting that because cyanide is a poison, not a drug.
But unfortunately for your situation, it's the only oral overdose that I think would be fast enough for you to get away with. Even death by the highly desired Pentobarbital needs between 1-4 hours to make someone CTB. While there is no true antidote for barbiturates, your parents could still attempt to try to save you with something like Activated Charcoal Powder to try to bind what's left of the barbiturate to itself so it can pass through your system without getting absorbed.
Since you have some experience with hanging, it might be worth reading the topic below for more ideas on how one can hang themselves:
Case: A 37-year old male was found dead in his home where he lived alone, with no signs of forced entry to the house. The body was lying supine on the dining table, with the feet hanging free and the head completely resting on the table. A thick, orange-coloured rope was wrapped three times...
sanctioned-suicide.net