SharePoint online question

Dear SharePoint online experts
 
Right now we want to use Office 365 only for collaboration ((Office 365 groups))
 
But for later on step, I want to move some part of our file servers data to SharePoint online and also our intranet
 
So I am thinking to take this approach
 

 
So I want to keep the first level to be only team sites that our MS Teams users can mount them on MS Teams and create channels under these teams no user will be able to create a team
 
But for the second level can be subsites
 
Note: because I know you cannot mount subsites on MS Teams
 
Now my question how can I manage to create one site collection and under this site collection create the department's sites with the ability to let the users mount their department sites on MS Teams
 
So I want to build the first level of the site collection to be teams sites and the users will be able to mount them on MS Teams
 
And I want to avoid building multi-site collection I want to build only one site collection
 
how can I configure this ?

Thanks