If you want to set up multiple mail servers then automation is your friend. Nowadays Ansible is commonly used for that purpose. If you are dealing with system administration you should have such a tool in your toolbox.
I have created an Ansible playbook on Github that you can download and use. Using it is pretty simple:
apt install ansible git
git clone https://github.com/Signum/ispmail-bullseye-ansible.git
ansible-playbook ispmail.yml -c local
By default the playbook determines your external IP address and uses the .nip.io domain to use for setting up your mail server. I just realized that nip.io does not handle IPv6 correctly. So I’ll have to switch to a more versatile domain like sslip.io soon.