I kinda can imagine they getting old like Granny Smith and still being friends and talking about their old days when they were all so desperate to find their cutie marks and that was such a silly thing XD
... Have you watched ANY of the Cutie mark episodes? A pony's cutie mar is and always is their talent. Also, Rarity makes dresses as a hobby. She got her cutie mark when she found gems for the dresses.
Her cutie mark didn't appear until the play. She didn't get the mark as soon as she found the gems.
Also, no, a cutie mark is related to their talent or discovery, not the talent itself. If we go by your proposed method, Twilight is good at sparkling, Fluttershy is good at butterflies, etc.
Twilight doesn't get her cutie mark because she's good at sparkling, she's sparkling when she gets into Celestia's school. Fluttershy's doesn't get her cutie mark for taking care of butterflies, she gets it when butterflies save her fall and she discovers the ground.
The cutie mark can also represent the event that occurred when the cutie mark was earned (for example, Rarity's talent is dressmaking, yet she has gems because that's what helped her realize it, as opposed to something dress-related like stitches). I don't know, maybe Applebloom built a garden or orchard to grow apples.