I’m going to have to agree WRT ATC and GRBL. I made some progress with my ATC changer, and it certainly makes manual changes easy, but GRBL really doesn’t have the infrastructure available to support tool changing, and none of the gcode senders really do either. I started adding ATC support into CNCjs, and made some progress, but the whole thing was turning into a giant mess. In the end, I think it’s going to be less work (and cheaper) to switch to something like LinuxCNC which has built-in ATC support.
GRBL is cool, but it just isn’t flexible or expandable enough to support tool changers in a sane way. That’s not to say that it can’t be done, but the end result is going to end up being a hack, and it’s not really going to be something that I’m going to want to live with.