Is there any way, once Virtualmin is installed, to change back some of the configurations that are asked during the install from ‘daemon’ to ‘on demand’ (like, for example, spamassasin) , so I can reduce the amount of memory my server is using?
Yeah, disabling services that you don’t need is a great way to have more RAM available for services you do need.
You can disable services in Webmin -> System -> Bootup and Shutdown.
For things like SpamAssassin and ClamAV, you can then go into Virtualmin -> Email Messages -> Spam and Virus Scanning, and set them to use the standalone scanner, rather than the server-based scanner.
If you’d like to be presented with those config questions again like after the install, you can make that wizard re-appear by editing /etc/webmin/virtual-server/config. Look for the line “wizard_run=1” and change it to “=0”.