DEVONthink should remember the position of the windows that were open when you quit the application. Beyond that, you could set up a window how you like it, then use Go > Workspaces: Add, to recall it.
Thanks for your mail.
-No, DTP does not remember the position of the (main) window when I quit the app, and turn off the Mac (otherwise, quite obviously, I would not have raised the question…).
-My problem is not that I work “with the same set of windows over and over again”, nor “recall them whenever” needed, I merely want that DTP open its main window once and for all at the same location, (the location I choose, not a default location as now) when I start the Mac.
Shall I understand that it is not possible ?
As I said, it is working as expected in macOS 10.12 and 10.13 here…
This is after rebooting the machine and launching DEVONthink.
You could try trashing your preferences to see if it resolves the issue.