So according to the weapon sling;

This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.

Range 30/120 Damage 1D4

Slow. If you hit a creature with this weapon and deal damage to it, you can reduce its Speed by 10 feet until the start of your next turn. If the creature is hit more than once by weapons that have this property, the Speed reduction doesn’t exceed 10 feet.

According to Magic Stone;

You touch one to three pebbles and imbue them with magic. You or someone else can make a ranged spell attack with one of the pebbles by throwing it or hurling it with a sling. If thrown, it has a range of 60 feet. If someone else attacks with the pebble, that attacker adds your spellcasting ability modifier, not the attacker’s, to the attack roll. On a hit, the target takes bludgeoning damage equal to 1d6 + your spellcasting ability modifier. Hit or miss, the spell then ends on the stone.

So assuming a +4 spellcasting ability modifier, would the damage with a sling be (1D4)+(1D6)+4, or would the spell supersede the sling and it become 1D6+4

Get three hirelings who take the stones from your hand and throw it for you. They throw with your Spellcasting Ability modifier, so no matter if they are the untrained for 2sp per day.

For 6sp per Day you make potential 3 * 1d6+4 damage extra per round.

If your party consists out of a druid, a warlock and an artificer, for 18 sp you potentially make 9* 1d6+4 damage per round, with a bonus action cantrip cast by all three of them :D