So, after some digging, it looks like the techmining_first_find group is responsible for this wild imbalance. All vanilla industry artifacts
except the synchrotron core/corrupted nanoforge/AI cores appear only in the rare_tech and rare_tech_low drop groups, but these three also show up in techmining_first_find, which as the name implies is only used by early techmining rolls. Tech-mining can occasionally trigger rare_tech_low, which is where the other vanilla artifacts are coming from.
It may or may not be an accident that only the pre-0.95a artifacts are easily acquired through tech-mining - I've asked Alex to weigh in on the matter and when I hear back I'll change the drop tables in accordance with vanilla's intent. Thanks for looking into this matter, Khornaar!
The next version will also include a method for adding "no atmosphere" and "low gravity" to planets, removing "irradiated," and removing rare ores from farming worlds.
Shameless plug: If you're looking for a lore-friendly way to increase your perfectly terraformed colony to size 7, a questline from my new mod
Beyond The Sector might be just the thing.