In short, slugs are not recommended in smooth bore 835 barrels.
The over-bored barrel could allow a slug to tilt or wobble on its way to the muzzle. At the very least this would potentially cause poor accuracy, or worse the slug tilts and gets stuck in the barrel.
Buckshot is perfectly fine, although I wouldn't recommend using anything tighter than a full choke. You'll likely get a better pattern using an improved cylinder though.
You can buy a rifled slug barrel for the 835, but for optimum results on target it will be limited to slugs only. As you said, it does seem like for your purposes the 535 might be the better, more versatile option...