I work on this project sporadically.
Some of my work on CQEngine is not publicly visible. It's used a lot in AWS and some of my time is spent supporting those use cases.
As you mentioned though, the project is stable. It is in production in a lot of organisations and I am not aware of significant bugs. I don't think it needs any major architectural changes.
That said, there are a few things that I am hoping to look at soon - for example integration with newer JDK versions. So I am planning to spend some time preparing a new release of CQEngine in the next few weeks.
Best regards
Niall