All supported OS are good for Directadmin. It also depends if you want to run PHP versions lower than 8.1 because if that is the case, you will need to choose an OS which does not have OpenSSL 3.0 yet, like Almalinux 8 for example, or a Debian or Ubuntu version which does not have openssl 3.0 yet.
As for migrating, it will take a lot of time anyway, but I would use the admin backup/transfer tool.
There are several ways to do a migration like this.
We normally first stop the Exim service so no new mail can get in.
Then make an admin backup of all accounts (if you have space enough on your disk).
Then transfer all accounts to the new server with Directadmin already installed and restore from there. Starting with first the admins, then the resellers and then all users to prevent issues.
Another way with a lot of accounts then something else might be better.
That is to also use admin backup/transfer and backup all accounts but without e-mail data and keep everything running.
Then transfer everything to the new server (or transfer during backup) and restore on the new server.
Then change nameservers for the domain.
And as last thing stop exim on the old server and rsync all mail to the new server or again user admin/backup to backup and transfer all mail to the new server.
It's almost the same, except that mail is not deferred.