Ashdar imperial especially should be doing this, but do not, is there a way to enable it?

A valid question but I am afraid it is not that easy

The reason I say that is the following: The issue seems to be present for all kinds of unique technology and late game tech: Raiders, Assault Marines, Enslavement, harmonization and in some extent also for some special mechanics like boarding and usage of Science modules etc.

This leads me to belie that the script(s) controlling the various AI empires is probably the same across all factions: this would explain why unique tech is not being utilized, nor mechanics that are tailored towards specific races such as the hiring of mercenaries

Of course it would be awesome if I was wrong on this and there would be a switch to turn these on but is unlikely.
A realistic wish might be that the script(s) can have the above elements incorporated with relative ease and the DEVs have made enough sales so far that the work can be done as preparation for future extensions

That being said, there are a few way you can help the AI utilize more of the assets it has been programmed to use: increase difficulty, decrease Marauder strength (see modding threads for more details), do not use loose cluster galaxy map generation and set pirate raids to the lowest probability (though I haven't seen these guys pop up for a long time now - but that might just be due to my setting