Okay, here's an idea that you might want to think about:
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Death Flag: In normal play, the PC's, being the center of the story, don't die. They can be knocked out, incapcitated, roughed up, beaten around, but when they get to 0 hp and fail 3 death saves, they're just in the DM's hands. You might wake up hours later naked and sore in a sun-baked inn room with no idea who is in bed next to you, or why they're bleeding so much. You might wake up in a goblin stew pot. You might find yourself groggily coming to as orcs pillage the village you failed to protect. It's up to the DM, but it'll probably be Bad News for your characters.
If you WANT to be able to die, you can raise the Death Flag when you fail a roll or get damaged. This instantly gives you a re-roll, or negates the damage, but it is a sign that for you, this is going to be Serious Business. Now, if you die, it will be permanent. But it's up to you to declare that.
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....and then go for the jugular in a TPK.
When the party wakes up fine and just has to deal with the consequences of their TPK, you'll have established two things:
1 - The Worst Case Scenario is not a big deal, because it'll be fun on the other side. Failure is fun, too.
2 - This is a cinematic kind of game with movie/comic-book logic, not a gritty dungeon survival game.
You'll also have probably earned some trust as a DM who isn't just out to bone them.
And then you introduce the cult of the Gozarian, and all will be good.