Would it be possible to turn leading a target on and off? Like a unit ability button? Or maybe ground fire mode stops leading the target and the other modes leads it?
If T1 arty had 2 different weapons, one with leading and one without, you could have a toggle button to switch between them. That's how the Cybran T1 MAA used to work before FAF fixed it.
The issue with this approach is that by toggling you can double the fire rate. I won't dive into the details, but it is a known glitch that applies to some units that - as far as I know for now - is not something we can fix.