Sombre,
You have a very intersting idea with that Warhammer wagon, and have found how to fire and move ! For my part i'll probably do two wagons, one clearly defensive with crossbows and a melee attack, and one offensive version with mostly melee weapons and maybe a short range ranged attack with low munitions just like javelins.
I will probably use idea you and Tichy suggest of disabling horses, will do some test.
Mine is for now returning to it's basic form only from form 2 (it has 4) and is eliminated at the end of battle in form 4 ( I consider it's too much damaged, crew is dispersed etc...)
In attachement you may see a very basic version of my full crew wagon, still needs improvements and original horses but it's better than using elephants for tests
