Mortar Halftracks are one of my fav. units by far - I find running two are more than enough to harass enemy positions. Barrage an area and leave, then watch the enemy wander around, trying to locate them.
I find the Tiger's more useful against infantry, myself. If I want something for anti-armour, I'll build a Panther or an Elephant
As for the relief infantry; they make good team-weapon capture squads, kinda like conscripts. I wouldn't use them for front-line combat, though.
Never tried the Opel Blitz, myself.