Is this homebrewed Enhanced Instrument infusion balanced

Question

To go along with a homebrew Artificer subclass that I've recently asked about that is very focused on playing musical instruments, I've created a thematically related Infusion, that any Artificer should be able to choose, starting at 2nd level when the Infuse Item feature unlocks:

Enhanced Instrument

Item: A musical instrument (requires attunement)

While playing this instrument, a creature gains +1 on ability checks related to their performance. Spells cast with this instrument as a spellcasting focus gain +1 to their spell save DC. These bonuses increase to +2 when you reach 10th level in this class.

Is this closely inspired by the Enhanced Arcane Focus, but sort of suspect that increases to spell save DC might be more valuable than a bonus on spell attacks (I expect the bonus to performances with the instrument is mostly a ribbon, and not important to balance concerns). The spellcasting DC bonus only helps bards and Artificers (either of my subclass, or other ones that infuse the instrument themselves and can thus use it as a focus), not other spellcasters.

Is this infusion balanced against other infusions available at 2nd level? Would it be better if you gained access only at a higher level? 6th, 10th and 14th levels have precedent, though the latter two only for the Replicate Magic Item infusion.

Answer 1

Likely unbalanced at 2nd level

Your instincts here are correct. Increases to spell save DC give roughly the same numerical improvement as increases to spell attack modifier, but the consequences of that improvement are greater. Many spells that require a save can take one or more creatures out of combat entirely; an additional 5% chance to do this is just more valuable than an additional 5% chance to hit with spell attacks.

This isn't too dangerous for low level artificers using this infusion for themselves. They have few effective combat spells that require a save, the main offenders being grease and faerie fire.

Bards are a bigger problem. At 2nd level, they have access to multiple spells which magnify the improved save DC by affecting multiple enemies (in addition to the usual single-target spells like charm person.) Their faster spell progression means that they'll pick up more powerful options faster than the artificer will. Hypnotic pattern and fear, in particular, get enough of a power boost when used for crowd control that you'll see noticeable differences after only a few encounters.

Try 6th or 10th level

There are not many examples to go by for this ability. Increases to save DC are rare, which should indicate that what you are trying might be dangerous. However, there is at least one infusion that indirectly raises spell save DC: replicating a Headband of Intellect. An artificer that chooses to use this infusion (or gives it to a wizard ally) could increase their save DC, and likely only by +1.

On one hand, the Headband confers other benefits (spells prepared, for example.) On the other hand, your proposed infusion increases save DC even for a caster who already has 18 or more in their spellcasting ability. This is why I'd suggest placing your infusion at 6th or 10th level (keeping with precedent for infusion levels.) I suspect that it's less powerful than the Headband, but perhaps not by much.

I'd recommend against keeping the +2 modifier at higher levels. The higher that number gets, the more unbalancing this item will be.

In general, the actual benefits of this infusion will be hard to see. If you want to know how powerful it is in practice, the only way is to try it out. If you do, you'll want to keep track of when and where it makes a difference. Knowing the percentage won't feel quite the same as seeing it in action.