These are all good resources, but the best of all is this forum. Because DT’s feature set is so enormous and its use cases so varied, structured resources such as manuals and online courses (another is DEVONthink for Historians, and on the manual side Joe Kissell’s Take Control of DEVONthink is quite brilliant as a stripped-down guide to core functions) will only get you so far. In particular, there are features not documented in any of the above that you’ll only ever find out about from this forum – such as control-option-command-O to open an Annotation file in its own window, which I use all the time. And users here are constantly figuring out ingenious hacks to do things that even the DT team themselves wouldn’t have thought of; the automation-heads here are a genius bar all of their own.
A couple of minutes a day scanning the Latest view for tasks, topics, and functions that could be of use to you either now or down the line is never time wasted; it might only be once a month that you pick up something life-changing from it, but those moments amply repay the time spent scanning. (My own latest “you can do that?” revelation was @bluefrog’s tip that you can turn an existing file into a document Annotation file by pasting its item link into the URL field in the document’s Info => Generic inspector. Absolute gold. You can even turn Word documents into Annotations!)
Here’s a tip for keeping tabs on these accumulated forum nuggets. Using the current version of the DT manual as your master document, give it an Annotation file in Markdown format (for easy link editing), and whenever you find a useful addendum on the forum, quote or summarise it to that file with a URL link to the original post and an item link to the relevant passage in the manual, stripping out the start and length parameters from the link. (This last detail will enable you to replace the manual with subsequent editions while retaining working links to the relevant passage of the text; see this post for explanation.)