Withdrawing OP token from Optimism

Your original idea will work. When they claim the token your contract will transfer the tokens from its balance to the user that is claiming.

Your contract is missing to define the token contract address to define the instance correctly.

TokenInterface _instance = TokenInterface('tokenContractAddress')

The function will be then:

function claimTokens () public onlyWhitelisted returns (bool){
                require(!_investors[msg.sender].claimed);
                TokenInterface _instance = TokenInterface('the_token_contract_address');
                uint tokensToBeClaimed =  _investors[msg.sender].invested * ratio;
                _investors[msg.sender].claimed = true;                   
                _instance.transfer(contractAddress, msg.sender, tokensToBeClaimed));
                return true;
            }

The conditional is not necessary because if something goes wrong during the transfer the ERC20 contract will throw.

You do not need to go through this hassle, you can use getAmountsOut on the PancakeSwap router contract to make this a lot simpler. You provide a path&amount of the first token in the path to use, and the function returns the token quantity you would receive at that moment. You can then use this data for price calculation.