Ra was the most invested in Earth, so it seems like he might know Tau'ri bits that the rest of the goa'uld didn't. And perhaps once he had the tablet he didn't care about being overthrown on Earth because he knew the Lost City wasn't there any more? (I'm sort of assuming the tablet was originally on Earth because—and this is where my lack of SGA knowledge comes in—I think Atlantis was originally on Earth and left from there, so it makes sense they'd leave a tablet there.)

I like the idea that Ra was researching for ways to ascend. I don't think there's any canon evidence, but if Ra was supposed to be so powerful, it makes sense that at some point he was on the same level as Anubis. (Anubis's history is basically that hundreds of unspecific years ago he "died" and just now has resurfaced.)

I really want to know if there are fics on this topic!