The latest versions of the Outlook app actually already have this new DND option "During evenings" wherein you can configure your after-work routine. By default, it is set to 5PM to 8AM schedule as you want in your question. But it can also be modified to fit a different routine. Here's the screenshot:
See Microsoft's offical website for the full details: https://answers.microsoft.com/en-us/msoffice/forum/all/outlook-data-hog/a88b5e3f-2982-4558-a190-1afa93044e1f
You can do a number of things to reduce background data usage:
Turn off mobile data when connected to Wifi.
This may sound strange, but apparently Outlook sometimes still uses mobile data, even ...