Decision Making in Strategy Game

So, well after dozens of failed projects I went back to the roots and finally there is an little Plaything which seems fine so far.

It is a turn based Strategy 2D Game, at the moment completly tile oriented.

Now I want an Computer AI.
And would need to know how to make decisions, especially in an bigger context and not only depending on the turn.
So, how to tell the AI to produce Units /which ones, or produce Technology, to attack etc…
And this, since it´s an Strategy Game, with many Units in one turn.

Of course I came to Behavioural Trees, but they doesn´t seem to fit, or I don´t understand them correctly.
Like, how can I say there: “Okay I have enough Pawns, now produce a Technology”.

When to attack and how to place my Units too…
And all of my Units have different Stats, another thing to care about.

Could you please give me some ideas /Examples how todo that?