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-bookworm-ansible.git cd ispmail-bookworm-ansible/ansible ansible-playbook ispmail.yml -c local
Please follow the hints in the README.md within the repository.