This was actually one of the really big problems my campaign ran into with our Bard. I took some time out this month to really dive deep into the bard class (not so much the subclasses yet), and it seems like especially the "String" specialty is almost purely made for Lore bard. Now, I know we all love Lore bards as the archetypal bard, but it's pretty ridiculous how useless this ability is without being a lore bard. Essentially you would only take it for Dimension Door, MAYBE Etherealness, and niche situations like if you regularly go around with a party of 10 people.
"Sound" spells are a little bit better, owing almost purely to the fact that Accelerando and Battlecry Ballad are incredibly powerful spells, and being able to double their range IS quite effective, but I agree with you that the list is quite short as it stands.
I honestly didn't really have a good way to fix this issue succinctly, and it ended up being a part of the many MANY issues I have with Bards in A5e, so I can't really offer advice on how to help you in your particular situation. Just to highlight what I mean I'll post a little snippet from the spell list I compiled while researching all of this.
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Honestly, I think Bard might be in dire need of a retune, it was basically single-handedly destroying my campaign both in and out of combat, and it was only thanks to my party's bard being a very understanding and generous person that we lasted as long as we did. We all agreed a lot of the mechanics felt either half-baked or poorly implemented, and didn't give much identity to the bard itself both within a developing game world and from a roleplay perspective. I ended up writing a framework for an entire class rework (or re-tune as it were hehe). We haven't yet play-tested it in combat situations, I am definitely debating if it's an issue that should be raised on the forums, but I guess for now it is what it is. I feel your pain
