I'd love to see Ukraine blanket Moscow with a drone swarm during that "parade". It would be crushing for morale.
I doubt Ukraine can pull off something like that. Mostly because Moscow airspace is probably one of the most defended in the world at the moment. A hundred of known echeloned air defense positions mapped by OSINT analysts in and around Moscow:
Now, even with all of those, yesterday Ukraine rehearsed their contribution to the upcoming Victory Day Charade:
Not sure how many attack drones were in this wave, but one of them made it to within 6.5 km of the Red Square, only to be stopped by a high-rise building.
So, to your point, I don't think an entire "swarm" of drones can make it all the way to the parade. Most will be shot down. But a couple of them might get through. That's still going to be enough to cause morale-crushing consequences though.
But then there is another issue: timing. Launching the drones from Ukraine, it will take them hours to get to Moscow, giving Russians plenty of advanced warning. They are keeping the time of the parade secret at the moment, but even if Ukraine somehow knew the exact time, Russians could just postpone it, knowing an attack wave is on the way.
So, slim chance of seeing long-range attack drones striking the actual parade.
That's why I was hoping for specops FPVs. You need a couple of drone teams renting a couple of apartments somewhere in Moscow. A few kilometers away from the red square will do just fine. Anything with a window from which to launch a drone will do. Equip them with the latest and greatest in Ukrainian fiberoptic FPV technology and you have yourself a way to slip past air defenses (too small to detect) and electronic warfare (can't be jammed due to being controlled by the fiberoptic cable). As long as no one cuts the fiberoptic cable during the couple of minutes flight time to the Red Square, you have yourself a way to eliminate a high-value target taking the stage at the parade.