I have just transferred directadmin server to server. It was, well, easy I would say. If you know what you are doing then it is.

Server had CSF installed as well.

First use DA admin backup on server A, transfer via ssh to server B, then use restore on server B from DA admin/restore backup. Use new IP address from the dropdown list. Backup CSF and transfer it to the next server (follow the path when backing up…correct folder).

Next, on the server A revoke Let’s encrypt for domain name server (IP you have used for nameserver): https://help.directadmin.com/item.php?id=2084
Then install Let’s encrypt: https://help.directadmin.com/item.php?id=648
Then this: https://help.directadmin.com/item.php?id=629

Then install CSF from here https://help.directadmin.com/item.php?id=527. Use option 3. Restore previously CSF backup.

Next, install custombuild plugin.

Next, I like to use this tool for installing PECL extensions.
https://help.poralix.com/articles/install-pecl-extension-for-php-directadmin-server

Next, enable forced SSL for each domain.