The soldiers would have been out of reach of Ukraine’s shorter-range ATACMS missiles
Shorter-range ATACMS still have a range of 165km (103 miles), why would a position 50 miles behind the front lines be unreachable for them? It’s likely that Ukraine is currently expending their existing stock now that they know more is coming.
Trying to demolish a capital structure like a bridge with rockets would be incredibly wasteful, they’re better spent for precision strikes on ammo depots, airports, vehicle storage, etc. The payload of the long-range ATACMS is only 214kg, somewhat on par with the FAB-500 bomb, which carries around 200kg with stated TNT equivalent of about 300kg. The truck explosion on that bridge last year was estimated at around 10 tons TNT equivalent, it barely shifted a couple slabs, and was fixed within weeks.
Using sea drones to take out supports might be a better idea, they’re at least considerably cheaper.