Is there any open source alternative to prefect cloud?

Not that I found. We had ideas to use only Prefect Core and do our own version of Cloud. As we got into it, there is so much goodness packed into Cloud that it would have cost us a lot more to try and build it on our own

there is now

Prefect has open sourced prefect-server (their cloud tech). It has a custom open source license that has has an anti-aws clause.