Here is the the official article from Salesforce.
Where in SF does NOT suggesting using existing TopLevelDomain as SAP domain!
Posting this article summary as answer, assuming the community finds it useful.
Existing domain: Use an existing, known domain (e.g., mycompany.com). This aligns the identity of messages from Marketing Cloud precisely with your brand's. It may preclude use of Reply Mail Management (RMM) and will mix the domain reputation of Marketing Cloud messages with messages from your internal mail system. DNS generally cannot be delegated to Marketing Cloud with this approach.
Note: Setting an existing domain (such as one hosting a corporate website) as a SAP domain is not recommended, as DNS records required by Marketing Cloud will likely conflict with existing records. If choosing this route, ensure you understand any potential DNS conflicts and that you are prepared to self-manage DNS.