As per my understanding and as you already know SharePoint Online Public Websites are discontinued by Microsoft, currently there is no way of using public site in SharePoint Online.
Instead Microsoft is suggesting to get a public website from a third party provider.
Why is Microsoft making this change?
As part of the evolution of the Office 365 service, we periodically evaluate the capabilities of the service to make sure that we’re delivering the utmost value to customers. After careful consideration, we concluded that for public websites, Office 365 customers would be better served by third-party providers whose core competency is public websites. Therefore, we’ve made the difficult decision to discontinue the SharePoint Online Public Website feature so that we can focus our efforts and investments on delivering capabilities in Office 365 that will bring more value to our customers.
Similar Questions:
- How to create a public facing website?