Clarification on GTM Resolver cache

Since in the new environment the GTM will be receiving iterative (not recursive) queries for the DNS resource records of the MYTEST.SUBDOMAINABC.COM, this means that the DNS servers that are querying for these records believe that the DNS server that their queries are going to is authoritative for the MYTEST.SUBDOMAINABC.COM domain.

Thus, certainly using DNS Express so that the GTM itself becomes an authoritative secondary nameserver to whatever backend primary nameserver is authoritative for the domain (and will receive and maintain the zone via standard AXFR/IXFR transfers replication) would be an excellent approach. (Alternatively, the GTM can simply act in inline ["middle man"] mode between the client DNS servers and the backend authoritative DNS nameservers, caching their responses [in configured Transparent Cache] for any such future client DNS queries.)