I managed to find them all, but I had one or two where I simply brute-forced the last rune. I found Poseidon most tricky of all.
I think it would be helpful if you would allow a tooltip to show the meaning of the rune on mouse-over. I don't think it would take away from the decyphering, but (especially in a short game) it's not realistic to start memorizing what each rune means. While going back and forth between UIs did give me a little bit more of a research vibe (which you might have been going for?), but it mostly distracted from the puzzle. I think having the tooltip mouse-over when inputting runes, but not when reading the tablets might be a fair mix.
Also it would be nice if you could forward the text in the intro (make it appear at once when pressing e.g. E).