I've got all the DLC available, but have yet to embark on them; I've been too busy completing the main Campaign.
However, unless the DLC is wildly different from the main game, I'd usually disband an early Anti-Tank unit that was assigned to my Core Units immediately...but if it were an Auxilliary/Reserve unit, I'd probably use it the way you are. I've found through hundreds of hours of old Panzer General games that non-self propelled AT units really only work to their full potential in a purely defensive strategy, like "Germany (West)", where they can effective stave off a River Crossing, and don't have to move far to do so.
Otherwise, their lack of mobility and offensive potential uses up a space that I'd much rather have occupied by something that can be where I want, at the time I need it there.
I kept it as reserve and pulled it out only for defensive scenarios or ones where I needed to anchor one point as everyone else moved on. Even in that limited role my AT unit ended DLC 40 with slightly over 100 kills.
Edit: I loaded my saved game and double checked that AT unit I started DCL 39 with. It had 272 kills by the end of DCL 40. Quite a bit more than I initially guessed. It became much more useful when it became mobile, but still, not bad for a unit only occasionally used.