Just a note about GCs - the Shenandoah GC is in development/appears to be currently usable.
Developed by RedHat, it’s designed for ultra-low (sub-millisecond, often 0.1ms) collection times, and is in many ways similar to ZGC.
Anyway, here’s one of Aleksey Shipilëv’s talks about it: Shenandoah GC: What We Know In 2018 with Aleksey Shipilev - YouTube
One note though is that ZGC uses coloured pointers, while Shenandoah prepends a pointer to each object - it seems likely (IMO) that ZGC will use less heap space, while Shenandoah will have better performance (for the application - not just for the GC).