Yikes! 30 Sep cut off? That's not a lot of lead time, given the amount of work database systems require to data model, benchmark and migrate. My apologies if this seems inappropriate but given the urgency, my employer, StarTree, has a free tier if anyone needs to try out alternate solutions.
https://startree.ai/saas-signup
I understand Rockset-to-StarTree (Apache Pinot) is not a 1:1 drop-in replacement. But hopefully it's a port in a storm.
Whether you end up on StarTree or another suitable alternate, I hope everyone has as painless a migration as possible. Reminds me a bit of how FoundationDB customers found themselves without a home when Apple acquired them back in 2015[0].