Yes with nginx and local dns record. This is possible with a local dns record and nginx setup correctly, in nginx you setup your local subdomain e.g. ha.domain.duckdns.org and the proxy manages the traffic to your ip:8123.
You have in between the docker vlan that may complicate things.
Here an example of tutorial but what you posted seems complete, I have little experience with OPNsense…
The part 4 of the tutorial you posted is what I am referring to, there putting the port 8123 should address your question.
Macvlan setup might be the issue if your setup is not properly done but hard to say, without more info on the setup.
Furthermore when you say testing with your phone are you referring to access from outside your local network?
Did you set the HA config properly to accept the connection?
Is something like point 9 of the guide Reverse proxy using NGINX.