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It looks like you have a good understanding of the procedure for installing Exchange 2019 in your environment. Here are a few key points to ensure everything goes smoothly:
Extending the AD Schema: You are correct that you need to extend the AD schema using an account that has the necessary permissions. In your case, you can use the ACC1 account in the xxx.com domain since it is a member of the Schema Admins, Enterprise Admins, and Domain Admins groups.
Preparing the AD in the Subdomain: After extending the schema, you need to prepare the AD in the m1.xxx.com subdomain. You can use the ACC2 account for this task since it has the required permissions in both the parent and subdomain.
Installing Exchange 2019: Once the AD schema is extended and the AD is prepared, you can proceed with the Exchange 2019 installation using the ACC2 account. Ensure that the account has the necessary permissions and that all prerequisites are met.