Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
You an add another e-mail address as an alias to your Microsoft account. This will make signing in a bit more easy.
However, if you want to get rid of the optus mail and actually move to a different mail for your Microsoft account, you will have to cancel your Microsoft subscription and then order a new one, using the preferred mail (for example, @gmail.com in this case).