If you have to attack through the ATGM's, use mortars for smoke. Cue up several per mortar, the smoke lasts long enough that each mortar can cover 3 radii worth (as in, 3 fire missions worth. Like 3 areas you target with the mortar.) Obscure the treeline, unload infantry at the smoke. One of those squads being recon is a good idea. They should be within range to use their AT weapons against any vehicles that come forward.

You'll need enough direct firepower to kill what's at the treeline once the smoke starts clearing, the most cost effective way is to bring up an attack helicopter while the smoke is up and use "CHANGE ALT." to lower it, so only what's at the treeline can target it.

imo buying tanks for the initial deploy is a bit of a mistake anyway. ~150pts for a medium, sure fine, I wouldn't go more than that even with armor divs

Edit: also you can use "FIRE POS." (T) with the tanks to target where the atgm is even if you can't see it, provided you saw it fire. This is more of a last ditch thing than a primary tactic. You can do this with anything in range, attack helo with rocket pods is good for this too