The powers that be say no because reasons.
I think it looks and works perfectly fine as shown from the many different angles but as always some nit picky excuse will be brought up as to why it's just simply impossible to do or add.
As it works in the mod right now it cannot move while reloading which is fine by me, if they bump up the cost to something like 200 fuel, keep the limit to only 1, maybe give it an automatic reload for both a QoL thing and balance so it can't relocate after a shot has been fired to balance it out, give it perhaps a longer reload time as well as a "lock down" ability similar to the PE Hummel where it for example can reload faster and has greater range but at the cost of a larger delay before it could move again so there's some sort of counter play there or something like that.
Basically my point is that there's a lot of ways to go at it and play around to balance it but yes, not gonna happen.
Anyway here's the link to the mod for whoever wants to try it out for themselves: https://steamcommunity.com/sharedfiles/filedetails/?id=811084352
It looks cool, but I'd never want to have that it CoH2.
We're just getting rid of an overly survivable Calliope that was "just" a bad medium in terms of defensive stats. We don't need a movable and precise B4 that is basically unkillable shooting across half the map.
Getting this to a spot that is even remotely "balanced" while still feeling like a Sturmtiger will be next to impossible. We already have Sextons and Priests and those are virtually unkillable despite them having a longer barrage and what is basically LV health and armor stats.