I did see your post, looks like you’re well on your way. What firmware did you end up running?
To add to your list, there are several ports of GRBL to 32-bit processors in order to add new features. GRBL on 8-bit won’t be gaining any big new features due to the memory limits. The 32-bit ports are built on the mature codebase, and are able to add new features for multi-axis, etc.
I don’t think I would put g2core in the “brand new” category, it’s been around for 4-5 years at least. It looks like that’s what Bantam Tools is running on their machines, so I would guess it’s ready for prime time (though it’s possible they have their own fork, I didn’t investigate much).
The problem with most of these forks is that they only run on a specific set of hardware. Sure you could probably get one running on their other’s boards if you really knew what you were doing. You just don’t have the support and stability of GRBL on atmega.
Another solid open source firmware is Marlin, though it doesn’t have all the good features for CNC routers. It has a ton of features for 3D printers and is under very active development. I use it for some non-3D printer projects running on 3D printer control boards at work and it’s a good option. They do have some CNC machining functionality, and I think the MPCNC uses it. I’ve been tempted to use it for upgrading my Shapeoko control board, but I’ll probably go with some 32-bit GRBL variant instead. Just looking for some solid hardware.
What I really want is a 32-bit processor and Trinamic TMC5160 drivers, a 36-60V power supply, and some larger stepper motors. Along with linear rails and possibly ball screws (although I like the speed of belts). Might get really crazy and go with one of the closed loop stepper driver controllers that Trinamic makes, Marlin just added support for those.