Short question: if I like apt updates to be carried out automatically, is it sufficient to manually change CONFIG_CHECK_APT_UPDATES in dietpi.txt to “2”? Should I use a script/function to change this value?

It is sufficient to change that manually. The daily cron job will then not only check for package updates (=1) but also apply them (=2). You may want to check whether the time of the daily cron job is fine so that the system is as idle as possible: dietpi-cron