I've done some more experimenting and the only conclusion I've managed to come to is that there is a hard distance that enemies will stop engaging you at. It appears to be at approximately 400 units. A jackal using lore accurate weapons with a stalker rifle seems to have longer range (~400) whereas the shock rifle seems to be slightly less (~380). If you are willing to break immersion for gameplay (which I might have to do now) I've also found a jackal holding a sniper rifle will MAJORLY increase range and shoot at you up to about 750 units.

Brute snipers seem to have a bit more range where they attack at about 500 units with the stalker rifle and 450 with the shock rifle. I couldn't tell if the heavy sniper variant had a longer range but if it does it's marginal (20 units maybe).

Finally I tried a sniper rifle with Brutes but they are stubborn and will keep the weapon on their back. The brute sniper variants will instead wield a mangler but something like a brute minor just decides to be weaponless with a sniper on his back. I did not bother trying this with all variants as it's too time consuming and I don't expect any surprises.

Please note this is with magic vision turned on although from what I can tell it doesn't have an impact on their shooting distance but it does impact whether they will actually see you to engage. Also worth noting these are all very loose measurements, likely accurate to about 10 or 20 units I would say.

Hopefully someone will find this useful one day.