I’ve confirmed that moving the limit switch has indeed restored my full Z-axis motion. Thanks to all who helped figure it out!!
As for the slow jog speeds, I read through the thread posted by @WillAdams. It looks like those settings must have already been incorporated into the default firmware (is that the correct term?), because running $$ from the MDI and checking the log shows that all my values are already at or above the “faster” settings mentioned in that thread.
However, the jog speeds are still really slow. Perhaps jog speed wasn’t one of the parameters that was tweaked? I don’t know enough about GRBL to know.
Any advice appreciated. I feel like I need to go get a sandwich every time I do a manual jog on this thing… 