Is the feeling of death unimaginable AND what exactly is 'the afterlife'?

The afterlife. Does it exist? Is it for eternity? What are heaven and hell exactly? All questions up for debate.
Being agnostic, I’d argue that the afterlife is a possibility, but we don’t have enough evidence to say that it’s a true or a false belief.
When you dream, a chemical (DMT) is given off in the brain (or something like that) and that chemical causes you to dream. The same would happen when you die. So the afterlife, in theory, would all be just a dream.
Before I knew what DMT was, I thought in definitely death would be this unimaginable feeling of darkness. Kind of like a coma, but still unimaginable.
If the afterlife is all a dream, wouldn’t that mean that heaven is a good dream and hell is a bad dream? If someone is the nicest person on Earth yet dies slowly and painfully in a tragic fire, could they go to what is theoretically, ‘hell’. And have a nightmare? And if the meanest person on Earth dies in their sleep, but was happy with their day, couldn’t they go to ‘heaven’ and have a good dream?
We dream in 5 minute intervals. Could the afterlife be just 5 minutes that seem like an hour? Is there an endless supply of DMT in our brains? If our brains are dead, they wouldn’t be able to produce DMT forever. I mean our brains shrivel up and decompose and whatnot after a while. DMT shouldn’t be functional forever.
This is all something to think about, up for discussion.