This small program performs a middle click on macOS - stick this in an Automator service and you can get a shortcut to perform a middle click!

middleclick.c

#include <ApplicationServices/ApplicationServices.h>

int main() {
    // get pointer location
    CGEventRef ourEvent = CGEventCreate(NULL);
    CGPoint point = CGEventGetLocation(ourEvent);
    CFRelease(ourEvent);
    
    // button down
    CGEventRef click1_down = CGEventCreateMouseEvent(
                                                     NULL,
                                                     kCGEventOtherMouseDown,
                                                     point,
                                                     kCGMouseButtonCenter
                                                     );
    CGEventPost(kCGHIDEventTap, click1_down);
    
    // button up
    CGEventRef click1_up = CGEventCreateMouseEvent(
                                                   NULL,
                                                   kCGEventOtherMouseUp,
                                                   point,
                                                   kCGMouseButtonCenter
                                                   );
    CGEventPost(kCGHIDEventTap, click1_up);
    
    // release the events
    CFRelease(click1_down);
    CFRelease(click1_up);
    
    sleep(1); // fixes dragging getting stuck - issue with CGEventPost not actually finishing the job in time?
    
    return 0;
}