Yes, that AI acts sometimes really stupid. But I think there are other things to do too:

1. AI is - as you said - developing only ONE type of weaponry - OFTEN the same as you.

Example: I play against a single AI (Drengin Empire). In one match, I focus on Beamweapons, next match, I'll go for Missiles. Our beloved Drengin Empire ALLWAYS mirrors me!

To prevent this, just add some kind of "preferred weapons" to each race. A typical Drengin-Weapon is the Missile I think, Altarians are using Beamweapons and Humans try to use Massdrivers. Its funny to have races preferring their OWN type of weapons instead of mirroring the player's tactics.

2. AI builds funny planets. I often found small planets with facs only, but no starport and so on. I often design my small planets to researchplanets to have the bigger ones free for Taxes & Building ships. But the AI is just building stuff like a nobrainer.

3. Altarians are almost human-like people. I found its really difficult to play with them, especially when playing as Human. They are often on "cool" relationship and there is no way to make'm friendlier - except having an empire a quarter the size of theirs. They're way to expansionistic, they often build up a HUGE empire with just a few small fleets. For Drengins, its really easy to destroy an Altarian Empire, just because its only HUGE, but really weak defended.

I think its better to redesign that Altarian Race: Less expansionistic, more defensiv, more an ally to humankind. At the moment, its easier to have Drengins on a friendly state than Altarians as friends.

Okay, thats it atm.

Actually there's more bugs in the tech trading than that.
Ever notice that the ai value certain techs WAY more than they should? A few early game examples I can think of off the top of my head....Xeno Research, New Propulsion techniques, Basic Logistics. I often play humans and use their diplomacy bonus to make up for the slow tech start they get. But those three in particular I found much better to simply research myself than to ever bother trading for, in fact you can trade for Ion Drive for less than what you'd pay for New propulsion...On the plus side if an AI doesn't have one of those I can often get a ton of stuff for one of them

Certain weapon techs are impossible to get by trading...my last game the Drengin absolutely refused to give up Harpoon...mentioned that I might use that upgraded weapon tech to wage war on him, makes sense...but when I ask for Harpoon II instead? hey yeah no problem....go figure.