I have a SharePoint Online site collection and I have a SharePoint group on the main site in that site collection that I would like to make a site collection owner. Is this possible, maybe through PowerShell?
I do not think so. Site collection admin should be Azure AD identity (group or user). It exists at tenant level. SharePoint group exists within SharePoint site.
Instead you should be able to create an AAD Security Group (login to https://portal.azure.com/, then Azure Active Directory, Groups, + New Group), wait ~10 minute, then you can
- manage group membership
- use this group in your site
Unfortunately for Sharepoint groups no, and this has been a long standing caveat with Site Collection Administrators - they can only be defined users.
You can use Azure AD / Office 365 Security Groups though.
There is no way to make a SharePoint Group as site collection administrator. As a workaround, you could make an offcie 365 group as site collection administrator.