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.

I think the premise that it's viable to "concentrate together when needed" is false. A key tactic on Ukraine's front line, due to the nature of the enemy and the nature of Ukraine's weapons, is dispersion. Concentrating force for any reason before Russia's overwhelming artillery has always proven costly. When that premise fails the utility of point defense like Gepard fails. The only way to salvage it is to imagine Gepards in quantities that a.) don't exist and b.) can't be crewed for lack of trained manpower.

I seek a larger force multiplier: a system that can effectively cover large areas by a small number of men at low cost. One Gepard could pay for many light fox hunting UAVs. With sufficient autonomy a large number of these could be operated from secure areas by small crews. The system should be capable of reaching behind enemy lines to preempt UAVs before they reach combat areas.

What you’re asking for isn’t possible since the development of non-gps & ai based navigation. Any system capable of launching physical strikes will be taken out by counterbattery fire almost immediately, and with the hardening against EW the drones have received you essentially can’t really do much without the equivalent of an EMP, which would likely cause you more damage than the enemy.