from my experience Sunders for eg. do not fire the Tachyon Lance if they are close to overfluxing. Only using weapons as such when they feel relatively safe with their shields, as they do not want to get overloaded.
I would assume same happens for Paragons.
If it comes to Tachyon Lances not firing at fighters. Are you really worried about it that much? If they are firing at fighters, there is most likely not much else to fire at. And the AI will hold fire if it wants to vent excess flux. So, it wont fire Tachyon Lances at fighters unless the situation is convenient to do so.
And if they are killing fighters when convenient, sure. Paragon doesn't care. But it does make the life of its allies easier. Obviously however it doesn't matter 9/10 times. But it can be helpful nonetheless. Like for example when you snipe Broadswords, Daggers, Longbows from very far away, allowing your Furies to push uncontested. Though obviously, competent High Tech ships which everyone spams have enough PD and shield coverage to not be worried about fighters. Different situation for Longbows though.
And if it comes to slow turn rate. I would honestly recommend using Advanced Turret Gyros on Paragon with Tachyon Lances. As Tachyon Lances' core weakness is slow turn rate. And resolving that makes them significantly more effective against lighter enemies. S-modding it makes them effective even more so... Dangerously so.
If it comes to wasting flux firing the Tachyon Lance at shields. You have to remember that High Tech ships that have the access to the Tachyon Lance tend to have significantly stronger venting stats than average crap you face off against. So, this only really applies to Remnants. Also, as long as the enemy has slight hard flux, there is always a chance the beam will pierce the shield and deal heavy damage to the target. So, the Dev did know that it would technically be a waste to waste beams on shields. So, they added an RNG mechanic that encourages you to fire them in spite of that.
Also, it's probably simpler to have the beam fire every time its available, because otherwise ships wouldn't feel justified having their shields raised. Than to have it only fire when the shield is down. Cause it could cause very... Visible complexities in designing AI behaviour I suppose. But this is an aspect that could be as relevant as the fact that sunsets on Mars are blue.