Using GSuite or Github to Manage who can log into Linux or Windows VMs on Azure
I would really prefer to not have to also manage a separate Active Directory Domain within our VNets


You've already found the answer: centralized authentication.

If you use a tool like Okta, you can indirectly tie GSuite to AD (or an LDAP server).  And then of course you can tie OpenSSH there as well.


  or deal with revoking ssh keys


Centralized authentication solves this.


  or updating multiple usernames/passwords on every single VM we deploy.


And this as well.

You've already got the answer, even if you don't like it.