Cannot add flights with large number of idle aircraft R0

I have about 2000 idle DC-6s. I took them all off their old routes so I could optimize my network.

Unfortunately now I cannot add them to any flights. When I am in the route details screen, not only does the page load very slowly, but if I select the frequency for an aircraft from the drop-down menu and select "add flights", nothing happens. The page reloads as normal, but no flights are actually added.

I now have 2000 aircraft that I can do nothing with because no matter what I do, I cannot add them to any routes. My only recourse is to use a different aicraft type that I have fewer of, and then manually change aircraft. But how many times am I going to have to do that? 100? 1000?

Doing anything in this game with > 200 aircraft at once rarely works for reasons that I cannot comprehend. Selling planes, buying out leases, all of these things tend to fail when trying to apply them to large numbers of aircraft at once. But this time it is legitimately impacting my ability to play the game as these aircraft are useless to me until I can add them to routes.