I think the millennium mark is a rough estimate. And Rykros was a failure on the Great Light's part, since it didn't warn Alis or Rolf. It only worked in PSIV, and even Rune was surprised to see it.
As for the Great Light predicting Dark Force's return every millennia, I don't think that there were any previous Dark Forces prior to when Phantasy Star started. Certainly the seal weakened every 1,000 years, but I don't think Dark Force was introduced until PSI when, as you said, Lassic summoned it.
It's kind of sad to think about. The Great Light left Algol and its children behind, leaving them to fend for themselves. Technically, their purpose is simply to make sure that the Profound Darkness doesn't get free, yes?