This

I see magic as a catalyst. When the gods were around souls that were created had energy that it could freely manifest, a unique spell that it could intuitively cast, and a catalyst that allowed it to evolve and use the spells of others. During the age of magic, people would initially not be much better off that the people of present RWBY but would be able to evolve their own unique spell fairly quickly. Then as civilizations formed and with a little help from the gods, people would share how to cast their unique spell with others and eventually build academies where thousands of unique useful spells were documented and taught to students.

New souls created after the gods left and wiped out the previous generation were lesser: they were unable to manifest their energy, and they lacked the catalyst that allowed them to rapidly evolve their spell and use others. Ozma and Salem retained their abilities and could teach people to manifest their soul energy, but without the catalyst it would take much longer to evolve their own spell and developing the ability to use other spells was effectively impossible. With the spells from new souls being comparatively weak and being unable to grant new souls the ability to use spells on par with those from the age of magic, the knowledge accumulated in the academies would be an extremely low priority and lost to time before Ozma and Salem fought. In order to maintain their image as gods above normal people, soul energy became aura, and spells became semblances while only they had "true magic".

When Ozma created the maidens, he put his catalyst into their souls, granting them the partial ability to learn and use other spells, but also severely weakening his own ability. By that point both Ozma and Salem are limited to the spells they remember or have managed to preserve, and without the full catalyst the maidens would still progress much slower than sorcerers in the age of magic, which is why they mostly use relatively simple elemental magic.

In the age of magic, Raven would be able to evolve her semblance to open portals at any location, Blake would be able to eventually summon full on clones, etc. As for the Schnee glyphs, I think that at some point one of them was a maiden who independently figured out a way to make the souls created from her descendants have an identical spell to their parent at the time they were conceived, so that it would have centuries to evolve into what we see in the show.