in my opinion the setting with one headless MoodleBox server and one or more clients will be the best. If you configure a Pi 400 as a workstation with an integrated server then you can’t migrate your MoodleBox to newer versions and you can’t use different sd cards with different Moodle content.
Using a separate MoodleBox server you can setup a new Moodle with new courses on every Raspberry Pi 3B, 3B+ or 4B. You copy the image to a new sd card. You start the MoodleBox, you setup new plugins and import your user list. You‘re ready to start with a fresh platform.
Using the workstation/server combination you must install the Xserver, the desktop and all the desktop Apps you need. And you must do this every time when you want to install a fresh platform.