A Steadicam is fairly massive and wouldn’t let you use the gun’s sight. Plus, recoil would knock it all over the place. Practically speaking it MIGHT be useful for the first shot if you hooked a camera up to it or something but from there it would be all over the place.
Making a larger version mounted to power armor might work however, especially if the gun camera linked to the helmets view. It would need to be a fair bit different in design, but some mechanical stabilisation is certainly a possibility. For now, we have the gyroscopic stabiliser gunmod which is probably the most practical thing you’ll get without making something massive.
Also, practicing with a weird stick stand is still probably going to be worse than just planting your own two feet. Those were made for when guns were 25 pounds and two metres long. Most modern guns aren’t more than half that.