Please leave the domain in Google Admin and migrate user data first. Please follow Google workspace migration prerequisites - it includes creating 2 subdomains and adding their DNS, so easier to have control over the domain parking and DNS.
Google Workspace migration prerequisites in Exchange Online | Microsoft Learn
The article below is overall guides through the migration. There are more articles to follow.
Perform an automated Google Workspace migration to Microsoft 365 or Office 365 in EAC in Exchange Online | Microsoft Learn
After we specify the migration path and migration type, we will be creating a Google Workspace project, a Google service account and giving it the required access. The automatic method will do most of it, but please also manually verify it
With a Workspace super admin, login to https://console.cloud.google.com. Make sure you're working in the root org.
- Select IAM and admin.
- In IAM on left-hand menu, edit permissions for organisation.
- Add Organisation Policy Administrator and save.
- Go to Organisation policies in left-hand menu.
- Search for 'Disable service account key creation'.
- Edit policy, set Enforcement to Off and save.
- Change workspace from root org to the project the 365 wizard created. Mine was called projectnamempij.
- In IAM and admin, go to Service accounts in left-hand menu.
- In the 3 dots menu besides the service account, select Manage keys.
- When in service account, Add key -> Create new key.
- The json file is created and downloaded.
- Create a new endpoint in Exchange Online and use the downloaded json
Migrate mail from Google Workspace to Microsoft 365 (manual method) | Microsoft Learn
Please follow the following article to create the CSV file for the migration. First column is 'EmailAddress' ending with ‘.onmicrosoft.com’ and the second one is ‘Username’ ending with the domain name in Google Workspace.
CSV files for mailbox migration in Exchange Online | Microsoft Learn
Please make sure to allocate required user licenses in Microsoft Admin. Please migrate the users in batches (multiple CSV's will be required). You can't add the same user in more than 1 migration batch. You will start seeing the errors after an hour or so, please be patient (note to self). Once you see it errored, fix the issue first and click resume migration.
Good Luck & Happy Migrating!