NextBSD ("FreeBSD X") operates in a similar space and was also looking promising, but it looks like development stopped.

In that approach, the BSD kernel was extended to support some Mach syscalls and libdispatch, launchd, and a few other userspace macOS-isms stuck around too.

Slides + Video: https://www.reddit.com/r/freebsd/comments/3iikry/slides_next...

There was also an effort some time ago to get opensource Darwin compiled and booted on ARM development boards:

https://github.com/darwin-on-arm