Hi Adithya,
As you are trying to do using MII Custom BLS then here is one option for you if you have one routing. You can create a custom MII BLS and within the BLS you can take inputXML in Transaction Input. From the inputXML you will get the StepId and Operation number. Now you can put a condition action block to check if the Step Id = <your last step id of routing> or by operation number if Operation = <Your last operation number> then you can use send mail action block.
Another option is - You can check the parameter "isLastReportingStep" value true from inputXML also if you have multiple routing or through the SAP ME PAPI service you can check SFC status "Done" then execute your mail sending action block.
The same BLS path you can put in yieldConfirmationRequest message Type in MEINT workflow configuration. It will send mail for the last operation of your SFC.
Thanks
Chandan