I’m sorry, but I don’t quite understand what your goal is…
NSD was added to automate and simplify the DNS configuration for the email domains hosted on Mail-in-a-Box, which is a pretty unique feature for a mail server appliance and the main reason why Mail-in-a-Box is so easy to get up and running compared to other appliances. Of course, this requires that users have the ability to manage DNS records for the domains hosted on Mail-in-a-Box if they want to use it for more than just email, which is why the Custom DNS section was implemented.
However, it seems that you want to turn MiaB into a full-fledged DNS and email hosting service provider platform, which it was never intended to be. Also, hosting DNS yourself has no advantages other than the already mentioned easier mail server configuration. In fact, rather the opposite is true for actual DNS functionality, or can a few self-hosted DNS instances e.g. provide ancyast? There are no privacy benefits either, since the DNS records are public anyway.
So if the DNS functionality provided by Mail-in-a-Box is not sufficient for you, you can simply use an external DNS provider of your choice, or if you just want to set up one or more secondary nameservers for the domains hosted on Mail-in-a-Box, you might want to take a look at my guide: Guide: How to setup NSD as a secondary nameserver for Mail-in-a-Box - #14 by miabuser