Balancing around Eldritch Cannon's use as a bonus action

Question

I am fairly new to DM'ing and I am going to be running a simple one shot soon. I intend to start the players at level 3 so they can enjoy the more entertaining class features and not be so squishy that the chance of a PC kill is likely while still presenting a minor challenge.

One of my equally new players wants to run an artillerist artificer and heavily use a gun-esque Tiny handheld eldritch cannon. Tasha's states that

"[they] can take a bonus action to cause the cannon to activate..." and "As part of the same bonus action, you can direct the cannon to walk or climb up to 15 feet to an unoccupied space, provided it has legs." (Tasha's, pg. 17)

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My question is would allowing the player to manually aim the cannon in their offhand in the firing bonus action be a fair equivalent to aiming the cannon through it's movement in the firing bonus action. If so how would be a decent way to balance the high damage output of them using a standard spell or attack in combination with the 2d8 damage of the cannon being fired with force ballista (Tasha's, pg. 18) as a bonus action and avoid dramatically outperforming other players.

Answer 1

This is already how the cannon works.

The rules for the subclass already allow what you describe:

A Small eldritch cannon occupies its space, and a Tiny one can be held in one hand.

An artificer holding the tiny cannon in hand can then use a bonus action to activate it:

On each of your turns, you can take a bonus action to cause the cannon to activate if you are within 60 feet of it.

You can select Forced Ballista when you activate it:

Make a ranged spell attack, originating from the cannon, at one creature or object within 120 feet of it. On a hit, the target takes 2d8 force damage, and if the target is a creature, it is pushed up to 5 feet away from the cannon.

The "aiming" of the cannon is the ranged spell attack - that's what determines if you hit or miss. So it shouldn't be a problem, as what you describe is how the subclass is written to work.

Answer 2

If so how would be a decent way to balance the high damage output of them using a standard spell or attack in combination with the 2d8 damage of the cannon being fired with force ballista (Tasha's, pg. 18) as a bonus action and avoid dramatically outperforming other players.

There really isn't much to balance here. A 2d8 damage bonus action doesn't cause the artillerist to dramatically outperform other PCs whose characters are good at dealing damage.

So, honestly, don't worry about it. The cannon can walk around next to the PC and shoot, can stand in the PCs's hand and shoot. And the game will work.

To wit:

A 2d8 damage attack is an average of 9 damage.

If they make a firebolt type attack with their action, this is a total of 14.5 damage (assuming things hit and don't crit) on average.

A player with two weapon fighting style and 16 dexterity does 2d6+6 or 13 damage with an attack plus a bonus action attack.

A player with a spear, polearm master feat, and 16 strength does 1d6+1d4+8 or 14 damage on average, plus can make a reaction attack when a foe approaches them.

A level 3 dual wielding rogue does 4d6+3 or 17 damage, with two chances to land the 2d6 sneak attack damage.

When you get a new class feature, often it gives a bump in power that puts you above allies. But the other players all got a subclass feature as well; if it didn't boost damage, it boosted something else.

The real trick here is that the 2d8 looks bigger than it is. Because spell attacks rarely add attribute bonus to damage, a 2d8 attack is similar to a 1d8 weapon attack.