A few things I've noticed about cozy mystery tropes after reading too many of them:
The amateur sleuth works because she isn't trying to be a detective. She's a baker or a bookshop owner or a knitting instructor who stumbled onto a body and can't stop asking questions. That reluctance is the engine. The reader trusts her precisely because she doesn't want the job.
Small towns in cozy mysteries function the way a locked room does in a classic whodunit. Everyone's a suspect because nobody can leave. But the claustrophobia is warm instead of threatening, which is a trick that's harder to pull off than most writers realize.
Joanna Fluke understood something about cozy mystery tropes for writers that I think gets overlooked. In Chocolate Chip Cookie Murder, Hannah Swensen's baking isn't decoration or a quirky hook. It's her way of processing information. She thinks through recipes. She kneads dough while turning over alibis. The special skill has to be how your character's mind actually works, or it just sits there like a costume.
The body always shows up at the worst possible time. The village fair, the grand opening, the Christmas pageant. This is partly practical because it gives the detective a reason to be there, but it's also doing something subtler. The murder disrupts the community's self-image. The rest of the book is the town putting itself back together.
Every cozy has a character the reader suspects immediately. The grumpy neighbor. The new person in town who won't say where they came from. These red herrings are a kind of promise. The reader gets to feel smart for being suspicious, and then the book gets to pull the rug.
The absence of graphic violence is doing more work than most people give it credit for. It's a contract with the reader. You can enjoy the puzzle without dreading what's on the next page. That contract is what lets cozy mystery readers consume four or five books in a week. They know they're safe.
Richard Osman's The Thursday Murder Club did something I haven't seen anyone fully replicate. He made the recurring cast of regulars the actual point. The mystery matters, sure, but you keep reading because you want to spend more time with Elizabeth and Joyce and Ibrahim and Ron. Osman once said in an interview, "I wanted to write about people who are underestimated, and then let them be brilliant." That line sticks with me because it describes what the cozy mystery trope of the amateur sleuth is really about.
The quirky sidekick exists to say the things the protagonist won't. Often they're funnier, less filtered, more willing to voice the suspicion that feels rude. A good sidekick isn't comic relief. They're the id.
I'm not sure whether the cozy mystery setting has to be a real place or whether invented towns work just as well. I've read good arguments both ways. M.C. Beaton's Lochdubh in the Hamish Macbeth books feels real because she drew it from actual Highland villages, and there's a specificity to the weather and the gossip and the particular way people are polite while being vicious. But I've also read cozies set in made-up towns that felt just as lived-in. Maybe the question isn't real versus invented. Maybe it's specific versus generic.