Your problem statement (protect mobile forces close to the front) is exactly the original mission set of Gepard and similar platforms (ZSU 23 for example). There's a reason they are tracked AFVs.

Ukraines current situation is that Gepards are more valuable in static defence roles, and they don't have enough platforms to employ them everywhere they want.

And you don't need enough AA sweep the skies clean. You just need enough to add sufficient friction to typical operations, and enough to concentrate together when needed to ensure l that you can locally sweep the skies when needed.