NFT smart contract with on chain auction

I come up with this demand in my project: https://phanta.space/#/space

In order to sell a vast amount of items with large variations in the utility value, I need to use an auction that initiates a countdown when the user places a bid and mint at the end of the auction.

Here is the implemented smart contract: https://etherscan.io/address/0x0e34dd8c227856ca59e2476dc2e87...