Hi,
This limitation for NetBIOS name come from Win NT era :)
You right, there is clear limitation:
https://video2.skills-academy.com/en-us/azure/active-directory-domain-services/tutorial-create-instance
Domain prefix restrictions: You can't create a managed domain with a prefix longer than 15 characters. The prefix of your specified domain name (such as aaddscontoso in the aaddscontoso.com domain name) must contain 15 or fewer characters.
To continue work with your environment you can try to install Azure AD DS with the PowerhSell and set the NetBIOS name manually.
In the last step instead of the full FQDN set it manually. For example:
$ManagedDomainName = "domain.com"
After you can add a UPN suffix into your AD DS servers same as a public one.