We don't currently use our default site and set it up to redirect to another site. I want to use it as the main site, so I am looking to somehow provision these templates or even somehow just create a new site with all the same web parts etc that these allow. Maybe I have to some how upgrade the site?
The default root site can be swapped for a different site in your SharePoint Online tenant. Therefore you could create the site using the look book and then swap the default root site for the newly created site using the guidance Microsoft have provided.
Yes you could replace the root site template using site design you need via look book UI or Powershell:
Refer to the similar issue for more information: