Here's a little hypothesis to work on;
The Thargoid menace attacked our systems all those years ago and as a result we created a way to disable their jump drives rendering them unable to go get reinforcements. As a result they couldn't gather resources either. So instead they crash landed into the planets and started about a metamorphosis into what we now know as Thargoid Barnacles, These barnacles once they are mature will start to produce smaller offspring and caretaker type units which scavenge and repair any wounded thargoid offspring. Eventually after a set amount of resources have been collected by the barnacle and its offspring a new metamorphosis occurs whereby the barnacle mass transforms into a Base. This Base then continues producing Scavengers and eventually starts seeding itself by laying eggs in its central chamber. These eggs are in fact what we see as Flowership's and once they are mature are launched from the top of the base into space with even more "barnacle seeds", these then start to deploy probes into the systems around searching for hostile threats and suitable planets for reseeding and extraction. The Flowership's take this data and relay it back to their home base, they also take any organic mater in any state and return to process it for further unit and base reinforcement. The response we see from the Flowership's now is purely a instinctive behavior learnt from the previous hosts which were persecuted by humans. I dont believe this is the final stage in the design of the thargoid's, i think that the base will transform once more into a war station and start producing soldiers. The reason that we don't see these current thargoid's flying off to get reinforcements is they probably considered inferior and weak by their original race and therefore must adapt and survive without a queen. However if the original race has come into contact then they are fully aware of the threat we represent as a race and are coming to eradicate us.
This is my hypothesis and i believe this is highly likely the correct chain events in the re emergence of the thargoid race.
That's an interesting theory!
I would say though that the ground bases we're finding are from the old Thargoids, they aren't new. They are damaged and there's what seems to be mycoid damage on them (from comparisons with the stuff at the INRA base and the crashed scout ship). The "eggs" are more likely to be blisters or boils, as a result of the mycoid virus. The Scavengers seem to be just left there dumping something on the wounded areas, it's likely this was an early attempt to heal the bases, I think they're just relics of the past rather than something newly growing (that's why several are dead, and have bits collapsed).
I think you're broadly correct though, from all the bits and pieces we know it seems like these Thargoids we're encountering now are "newly born" and are emerging from somewhere. Either they are emerging from the Pleiades or travelling there without interacting with anything else - there are no interactions with Thargoids outside around 150ly of Merope. I don't think the ships are "grown" from humans, since Thargoids are ammonia-based and humans water-based, and so to them we'd be super-hot organism that would literally burn them on contact. It's more likely they are either researching ways to kill humans, or implanting them with brain-slugs and returning them to the bubble as spies!.
It does seem that it's likely that their "home systems" are in the Col70 sector, although why there's been no Thargoid encounters around the borders of that area is unclear. From FFE lore the Pleiades was the Thargoid "Forward Staging Area". But that doesn't really explain why they seem to interested in that area and no-where else in the Galaxy (that we can get to). I do think the ships we're seeing now are animals, I don't think they have "pilots" at all, they're grown and are sentient.
The Barnacles do seem to be an outgrowth of something. We've seen the ships interacting with them and "extracting" something. We know the ships also seem to like Meta-Alloys when they're given them. It seems that meta-alloys are important to them.
I suspect that they are building something in deep space. A space-station or something similar, or a new fleet, or shipping the meta-alloys back to their home systems? They don't really seem to care about planets very much but we know they collected a lot of data on them via the Probes. It seems likely that the Barnacles are mines/factories, generating the resources the Thargoids need.