When I setup google admin I always do it automagically as I am bound to forget something.
There is nothing wrong with creating a servie account with full access and then choosing the api scopes and limiting a specific gam project to only a few. Then create a full functioning gam project that can do it all.
This approach is a little more open that your example (open as in more privileges and possibilities) but much easier to manage and maintain.
Just my $0.02
--
Rance Hall
Application Specialist
ESU 10
Some days are better, some days are worse.
Look for the blessing instead of the curse.

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Subject: [GAM] GAM Read/Write Files in Specific OU
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