Hi Denis,
As far as I understand, none of this is directly supported by Geany right now.
This is a known problem: https://github.com/geany/geany/issues/1458
Until it is solved systematically (if ever), you might be able to hack around it.
For custom autocompletion, see: https://github.com/notetau/geany-jedi-complete/tree/master/geany-complete-co...
For tags, you do have access to parts of TagManager such as tm_workspace_add_source_file -- maybe you could use that creatively somehow.
And of course you can have a custom "Go to D Symbol Definition" command.