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Pedantic Grognard
Well, from that thing I've been working on that I occasionally mention in comments, I've got two obviously theme-appropriate Eldritch Invocations for a warlock with that concept:
Whenever you complete a long rest, you may replace your choice of musical instrument with another.
(Yes, yes, optimizers, these are obviously not EIs you take unless you're doing a social-focused campaign. In a dungeon they're worthless.)
Entrancing Melody
You may cast charm person without expending a spell slot by playing a musical instrument you are proficient with for one minute.Preternatural Maestro
You gain both proficiency and expertise with one musical instrument of your choice. This means your proficiency bonus is doubled for any ability check you make with it. You may use a musical instrument of the type chosen as a spellcasting focus.Whenever you complete a long rest, you may replace your choice of musical instrument with another.
(Yes, yes, optimizers, these are obviously not EIs you take unless you're doing a social-focused campaign. In a dungeon they're worthless.)