As described in Process for reviewing classic confinement snaps, the direct use of sudo and configuring systemd services are unsupported use cases for snaps and are not candidates for classic. I’m not familiar with the technical implementation of dnsadblock-desktop, but based on the description alone, opening a port, updating the resolver configuration and controlling network-manager is all supported via network-bind, network-control and network-manager. snapd also supports daemons in snaps so writing out systemd files is not required.
You mentioned that your snap just wraps something else that does these things, but, unfortunately, as documented in the aforementioned URL, this also isn’t justification for classic.
It sounds like your snap needs to be rearchitected a bit to work within the snap ecosystem. Without making these changes, your snap has many of the same characteristics of the unsupported “management snaps” category. You may want to visit and/or create new topics in the snapcraft category for helping transitioning away from classic.