The Guardians didn't use AIs without concern though. The war that wiped them out was fought over the AIs. So, just to outline in brief:Well, i'm personally more on the road that thargoids are the descendants of the guardian AIs, so to speak.
Look at it in this perspective:
We know the guardians could alter DNA, create living beeings for special purposes.
They were realy advanced in biology and it was no shame to use it.
In their future, they created AIs, used them with no concerns. The technology group linked itself with them, used global networks etc. (It's said that Thargoids have some sort of colective mind > maybe a development step from the network. It was their way to communicate, the best way. Why not use it in another form, more advanced and independant.)
But after some time the guardians split in two groups, fighting each other. The AIs were teached from this war that the hosts, the guardians, are to vulnerbale and they created their own bodies to exist without a need of a vulnerable host/implant.
So my personal thought about the thargoids:
They ARE the AIs, or what they are now.
They destroyed the guardians entirely, maybe. Because ALL of them could be a potential threat to them. Or the old story: Live is imperfect, makes mistakes, emotional (religious), to slow in developement and a threat because fear and so on.
It would explain why they act so passively, when you don't harm them. They doing their job. We are not of interest, for now and in this moment. Effectiveness all in all. Like an AI would act, like an AI "learned" to act.
They search something, they need to destroy a ship for that? They do it. They see guardian technology? They want to destroy it.
That would be very terriying.
-Basic AIs were created
-Guardians developed brain implants which allowed them to connect themselves directly to their network.
-The Implants provided extra processing power and allowed the AI to develop sentience.
-This resulted in an exponential increase in the development of science & technology
-More Guardians got implanted to keep up.
Guardians society then split roughly along the lines of those who were implanted and those who didn't accept the new technology. The latter became a religious movement which eventually exiled all of the Guardians who had accepted the technology.
The AIs at this point were still virtual beings that only existed as part of the network. However they wanted to be able to exist independently and sought to achieve this. The religious group turned to violence, not just to prevent it, but also against the Guardians who accepted the AIs.
This led to the second civil war which ultimately wiped out the Guardians. (Although there's also the 'Foe/Adversary' which may have been responsible for their final eradication.)
Anyway, on a few of your main points...
Yeah, a FTL network of minds could potentially be the basis for a collective mind. There's also the exiles to consider here- we don't know what happened to them, but we do know they were implanted, and hence could also connect directly to networks, and also act as hosts to AIs.
If the Thargoids were the Guardian AIs then major parts of Guardian society embraced, supported and fought for them. Why would the AIs act against all Guardians?