Basically you cannot create a Super Admin group, and there is a reason:
Super admin accounts have irrevocable administrative permissions that
we do not recommended using in the day-to-day administration of your
organization.
Indeed, this does not really fit with a group-based policy.
Super admin accounts must be managed with direct ownership and countable.
The best practice advised by Google is the one as follows:
Create a new email address that is not specific to a particular user
as the G Suite or Cloud Identity super admin account. This account
should be further secured with multi-factor authentication, and could
be used as an emergency recovery tool.
Disclaimer: Comments and opinions are my own and not the views of my employer.