Post patch, most artillery feels way less harmful than before in my opinion. If you place your OKW truck so aggressive that it can be destroyed by indirect fire without the enemy having to push then it is your own fault. I usually place my Medic HQ in my base (in 1v1) and the Flak HQ at the fuel/cutoff to defend it.
The Flak HQ didn't get an "extra anti air ability" - The ability to target airplane is now just toggable with a 30 seconds cooldown. In this time, the flak WILL NOT shoot at infantry. So it is now either/or and not, shoot down the planes and then immediately fire at inf again.
The whole point of trucks is to have them be more rewarding if you place them outside your base.
But right now its only somewhat useful for panzer HQ and large artillery pieces have more than enough range to safely barrage stuff even in your base, so what's even the point.
Also no, it's not a togglable mode. It's a 60 muni ability with a 1 min cooldown during which pz HQ can't shoot anything. And honestly it sucks for what id does. Who even decided that out of all factions it was okw that had to pay premium and have drawbacks for their AA when everybody else gets it fot free as a side bonus for their specific units?