I'd doubt that a Avant would have enough lift for a bag filling bucket there quite heave and you need to lift more than a full bag of material.
If you get one get one with chains on the doors rather than the rams straight to the door sand gets on the rams and eats the seals in no time.
The biggest Avant will do it lift capacity wise.
But I could envisage two problems.
1 is lift height. Depending on the bucket design you might struggle to stack them 2 high or load straight onto the back of the van/lorry. This is where the time saving really kicks in.
Second problem is the geometry of the bucket means they take a lot more power to load than a normal bucket. I've only used one on a JCB 2
520-40 and it struggled to fill the bucket with type 1. Would spin the wheels. And it's twice the weight of an Avant. Ballast and shingle are fine.
Best ive seen is a hopper over a frame that had 2 upturned u channels so a bag could be hung on them and then lifted off with a forklift.
One person loading the hopper. One person hanging the bags on and operating the hopper chute and one person in a forklift stacking them.