Ok, so the more I tested and the more complex the applications became (beyond the thousands of tests), it became pretty clear that the Immix-inspired GC was either super fast or super slow. The whole STW and synchronization kungfu really dragged this down and made me question my architecture.
Good news is that I found several interesting research papers and that gave me some ideas for a complete overhaul. We have now a MaPLe-inspired Hierarchical Heap GC (per-fiber heaps, depth-based write barriers, Cheney copying collector, TypeDis extensions, …) and we no longer have to pay the costly price of waits & pauses.
It actually works soo well, that I dont quite trust it atm. But still happy to share the progress here. Take it with a grain of salt; Seems like we play in the league of Rust frameworks now?! 
I guess I will setup the TechEmpower Benchmarks soon to get a clearer picture.
Here you can see the distribution of collections:
