A lot of these guns are ATGs and not artillery, certainly THEY had iron (open) sights or optics that could be modeled. And whoever said the tank sights don't work are right. The demarkations mean nothing, there's no range scale (if there is it isn't accurate).
Artillery field guns did have open sights. Well, at least the howitzers a little bit after the war did, so Im assuming on this one.
You cant totally remove the crosshair, then you have no idea on how to adjust fire at all!
Just add a different sight closer to that of the tanks.
Besides, removing the crosshair doesn't disable pixelaiming. There are still a plenty ways to do it.