Preparation for the self-construction of a carousel changer

1kHz is more than can be done on the GPIO with software PWM, which was my first idea.

The 7i76E has a number of expansion ports. What (if anything) do you have plugged in to those? There may be one or more PWM channels available there.

If you start LinuxCNC from the command line (command "linuxcnc") then there should be a dump of all the pin functions printed to the terminal.
See if you have any extra PWM channels showing.

You can either connect directly to the pins (though it's a bit risky) to get a PWM, or attach any of these boards and take the PWM from them:
store.mesanet.com/index.php?route=product/category&path=83_87
Don't be too concerned about the intended use of the boards, a step-dir output can be used to output PWM if the pin assignments match. But the cheapest board (7i75) is purely buffering/protection and doesn't need a fortuitous match.

Or 7i75.

Which firmware do you currently have on the 7i76E?

I have been poking about in the hostmot2 firmware files for the 7i76E and found a firmware that swaps one of the step-dir channels on the 7i76E for a PWM / DIR channel.
Are you using all the stepgens?
It's the 7i76e_7i76x1p2d.bit file

What is your current "loadrt hm2_eth" line in the HAL file? It might be disabling PWM on the current firmware.

Do you have Mesaflash? (You are likely to need it soon)

sudo apt-get install mesaflash