Should I add subdomains to the dashboard separately?

The need to add subdomains to the platform separately from the root domain ultimately depends on what email sending practices the subdomains will use.
It’s important to note that the root domain’s DMARC policy is inherited by the subdomains. This means that if a root domain has a DMARC policy, the emails sent from subdomains are also going to be treated similarly to those from the root domain. The subdomain’s email activity is also presented in the DMARC reports of the root domain.
Even if you want to set a separate DMARC policy for the subdomains, you can specify it in the root domain’s DMARC record with the “sp=” tag.
For example: v=DMARC; p=none; sp=reject

Refer to this article for more comprehensive guidance about setting up DMARC records for subdomains separate from the root domain.

However, if subdomains follow different email-sending practices—such as using separate providers or sending distinct types of messages—you might consider adding the subdomains to the platform separately from the root domain to have better control over subdomains and monitor their sending practices separately.