At minimum you will not want to miss on a viable thunder event eidolon. That means you absolutely do not want to miss out on Amaru, who is either the 4th or 5th event after Mastema/Kyuuki ends.
If you miss out on Amaru, there's basically either a ~3.5 month gap until Phoenix or ~6 month gap until Tartarus for the next event eidolon that's viable to serve as the main slot for a thunder team.