Updates to SharePoint self-service site creation

Heiko Fuhrmann - for migration - it really depends on the scenario, though we recommend moving to modern sites (whether group-connected or communication) if possible - as these already reflect recent updates and feature investments (as opposed to classic site templates - which are still available in SharePoint Online and have many of the modern site features).

You can "groupify" a classic team site, but no, there is no mechanism to decouple a group from its associated SharePoint site.

For identifiers, you can use the template titles available when you use a command like Get-SPOWebTemplate. Hub isn't a template, but a setting on an existing one; these can be identified via Get-SPOHubSite cmdlet. You can find the full list of available cmdlets - even PnP ones - at http://aka.ms/sppowershell. 

I'm not sure what you mean about default quota per site type. With the new pooled storage model, admins can set the limit, but only the actual storage used counts against total pooled storage - regardless of template. 

A group-less modern team site has all the same SharePoint site elements and features as the one connected to a group. All of the related group-elements, like the user membership, shared mailbox or Yammer group, and related workloads (shared Planner, etc.) are not available. HTH!