From the pattern I mentioned September is still in the 5-3 month range the other graduations happened in, since they debuted in December. If they do it similar to Pomu/Kyo, they would announce Kotoka (or someone else) in the first week of September (two weeks after Hex).
I didn't mention Kunai and Vivi in the rrat, because if the contract expiration time is true they should have already graduated some time between May and July. Although Kunai's last stream was in May 11 and her hiatus was announce in July, so its possible she is effectively graduated and are just waiting to announce it. Especially now that they turned off her memberships a few weeks ago. Vivi is still active though, so she might not be leaving?
Although now that I checked, all three in their wave graduated in PL on the 19-24 of September. Maybe that's around when they signed/will expire the contract, but wouldn't expect any graduation news from them after that. But their graduation would be way closer to debut than anyone else so far.