https://roll20.net/compendium/dnd5e/Weapons#content This page is your friend.
Loading: yes, you can only attack once per turn
Heavy: small creatures have disadvantage to attack rolls (basically a -5 penalty to hit). Damage should remain the same. This doesn't affect Dwarves as Dwarves are medium creatures
You could also use a light or hand crossbow. Or any of the heavy weapons, but with disadvantage to attack rolls.
Proficiencies don't stack.
Of course, all of this assumes that Larian will implement these things rules-as-written.
