I landed up creating https://github.com/atmonostorm/derecho to answer the question of "in-process temporal.io". Works pretty well, with some unique features (error-controlled retry behavior) that temporal can't do.

I've used it pretty heavily in production, ~30m workflows of various types from various projects through it at this point. No capacity to support it as OSS though, and my internal persistence backend isn't easily extricable from monorepo, but it's just an interface with conformance tests.