As of now, using Microsoft Outlook for Mac 16.88, I think the most efficient way to save emails from Outlook to DEVONthink is by printing (⌘P) and then selecting “Save PDF to DEVONthink 3” in the drop-down menu on the bottom left corner of the print dialog box. It is quick.
If more than one email is selected, the selected emails will be merged into a single PDF.
For my purposes the output is certainly usable, and most importantly, searchable.
If someone is aware of a better or different, current solution, I would be interested to know.