Am I required to migrate to Australia on the initial entry date?

You must travel to Australia before the expiry of the initial entry date in order to activate the visa to its full term. Migrant visa holders are not required to migrate / settle permanently to Australia before the expiry of the initial entry date. But you (and any members of your family who is included in the application) must at least make a trip to Australia before the initial entry date your visa is then valid for return travel until the final expiry date.