dont think there is off the shelf solution to generate the report about number of groups getting created, there are third party reporting tools which you can explore for sure.
regarding your question about triggering an access review automatically again the answer is unfortunately is no.
now about best practices for creating o365 groups, there are basically two approaches
you throw it open to the users where in they can just create groups from outlook or SharePoint, which most of the orgs use these days as it gives a breather to administrators.
but if you want to give the group creation control to users then I would suggest that you create a simple form in SPO tied to a flow ( which can include necessary approvals ) which in turn triggers a logic app or azure function to provision a group for the request.