I haven’t used a C3D, I own mostly genuine Smoothieboards - 3XC and 5XC.
Oz developed the Cluster mode extensions for Cohesion a long time after I did my last machine upgrade, so I haven’t used it.
I rarely engrave - my business is nearly entirely cutting, and my K40s were used in my former tech hub, both as a basis for teaching how to build, configure and upgrade lasers and CNC routers, as well as teaching materials technology and general digital manufacturing.
When we moved from Asia I only brought one K40 with me, new in box, which has remained in its box 
The C3D forum is very informative and may prove useful.
As to the controller, it’s like anything in this tech arena, there will always be something better, but for my money it bridges the gap between 8-bit controllers and DSP controllers nicely.
Their Kickstarter offering start at €65, I think, for a complete unit with drivers, so not break-the-bank expensive.
I’ve probably got a couple of dozen controllers of various sorts from Arduino+shield to raspberry pi+hat to ESP32 to Smoothie and a bunch of RAMPS, some home-built (I’m an EE, by profession) and odds and ends of Chinese knock-off, but the one I always favour is the smoothie, despite the eye-watering cost of the 5XC
One other strong recommendation is the TinyG/G2Core.
It also has advanced sine-wave motion control and some cool features. The TinyG is more suited to CNC or cutting as the 8-bit ATMEL microcontroller is too slow for engraving, but the G2Core firmware using a 32-bit Arduino Due with their gShield or a hacked cncshield (hacked to make it work with 3.3v vs 5v - easy mod) makes a cool little controller