Cannot expand site group users without administrative rights

I've got a problem with SharePoint Rest API.

I've got a service account with "Full control" permission level.

I need to list all group members in my site collection.

I can run:

https://mytenant.sharepoint.com/_api/web/sitegroups

It returns my groups in the site collection, but does not have their group memberships which I need.

So I try the following, but I get blocked trying:

https://mytenant.sharepoint.com/_api/web/sitegroups?$expand=Users

I am, however, able to get the members of each group individually. Example:

https://mytenant.sharepoint.com/_api/Web/SiteGroups/GetById(6)/Users

This is now going to force me to do NumGroups extra API requests on SharePoint which isvery costly for me in terms of rate limiting.