I thought about building an API but my current plan is a bit simpler:
(1) Upload the Devonthink “Metadata Overview” to Retool as a spreadsheet. That will let me easily build a front-end to display all DT3 metadata and custom metadata in whatever format I prefer.
(2) When I want to display actual documents in retool, I will take the corresponding x-devonthink link and use it as a parameter in a Keyboard Maestro URL trigger. In turn that will let me run an Applescript to retrieve the desired document.
Then I can use Applescript (or more likely Python) to upload the document (or a folder) to Google Drive and also upload to Google Drive a URL link corresponding to the requested document(s). Retool can then display the content of that URL in an iFrame. Thus it should be possible for the user to be able to select any desired document or Group in Retool on the web and retrieve the document from the Mac which runs DT3 without being aware of the details of how it is being done in the background.
(3) If I want to upload a document from the web to my Decos server, then I can use whatever front-end I wish. The back end will use the SMTP email service in Retool connected to my existing “Utlimate Email Import” script: