Well... you can also do that from within the panel. Clear nameservers in the ip manager, also you can remove the ns1 and ns2 A (and AAA) records from the abc.xxx domain.
However, maybe your solution works too.
It's normal that if you create your own nameservers that domains get the domain used for the nameservers NS domain records.
For this reason you have to think which domain you want to use for your nameservers.
Also reseller can use their own nameservers on their domain and their customers then get the reseller's NS domainname records.
I don't know if you use external DNS now again. But I would suggest starting your own nameservers again if possible, because that would make life a lot easier for you. Also for wildcard dns (no lego needed anymore), all records are instantly updated so you don't need to copy them anymore to external nameserver.
You can use your own nameservers for all domains also for your customers, so your company name is visible in the nameservers.
However, it's ofcourse your free choice to use whatever you like.
