Sorry, I am not familiar with the BSC-Chain, maybe you should ask for help in their forum: Home | Binance Chain Forum
And they have a documentation about how to verify contracts, maybe you can have a look at it:
You have flattened your contracts, maybe you can have a look at this tutorial: Verify erc20 token on etherscan that was deployed through remix:step by step guide
And if your original contracts are multi-files, I think you can have a look at this tutorial: Verify smart contract inheriting from OpenZeppelin Contracts