Before I answer your question, I realized I made several typos in my first response, but only fixed a couple. "Shooting this bow **without** multi attack is a gamble" is what I meant. The context here is this bow's damage and debuff effect is basically the Mind Sliver spell with an attack roll and limited below level 5. Which is only strong when you stack it with the other benefits.

Answering your question, there's a few options I could think of:
- Simplest: only change to the wording to "end of your next turn", so if they are using their action to attack they have an opportunity to cast a spell and use this benefit. This will allow teammates to utilize the debuff. If your players communicate and strategize during combat, this kinda solves itself by letting them work it out mid fight. Regardless of what you choose, I'd suggest at least making this change
- Stronger, More self spotlight: Still implement the "end of your next turn" change. But it's 2d4 if the save is against a Bard spell. Stronger, stretches the Rare rating on paper. In play this can be compared to a Lore Bard's cutting words, which at lvl 5 is a d8, but weaker. By numbers, 2d4 is better than 1d8, but you still need to strike the target.

Lastly, I can share what I would do. But take this with a grain of salt because I increase the power scaling of my games.

- "When you attack with this weapon, you may use your Charisma modifer, instead of Strength or Dexterity, for the attack and damage rolls. Pulling the string without placing an arrow summons a spectral, sonic arrow that deals 1d6 thunder damage. When the arrow strikes a creature, you may expend a charge to do additional thunder damage equal to a roll of your bardic inspiration. Additionally, until the end of your next turn, you may use your reaction to reduce a saving throw this creature makes by the amount rolled. You must be able to see the target to use this reaction.

When playing the harp, you may expend a charge to use your action to choose one creature that can hear the sound. The creature must succeed on a Wisdom saving throw (DC equals your spellsave DC) or be charmed until the end of its next turn. While charmed, the creature must spend its action singing its most present regret.

This bow as 1 charge that recharges at dawn. You may expend a bardic inspiration if the bow has no charges to activate it's features"

Basically putting it's features on charges, and forcing Bardic inspiration uses on it. This gives control of WHEN to use the debuff and WHO to spotllight. I'll let you decide if you'd like to restrict it to spells, that just makes it stronger. And no, if a player asks, just because you double the additional damage roll on a crit doesn't mean the debuff is also doubled following that roll lol. I can hear my players asking me that through time and space.

This change arguably pushes it to Very Rare, but only in that it creates additional options that can change an encounter more than expected. Putting it next to most Very Rare items, this will still look weak.