Max charisma will certainly impact it. I have no idea what your race is of course, but the 23 for humans/halfs is always going to do better than a wood elf (22) who is better than high elf (21) who is better than arial (18). Most bards always have charisma maxed though, but never hurts to be sure. And comparing your effecicay to other bards is not always easy, unless you know for sure all the details about them.

It does not hurt to have a back up instrument if you want to switch repertoires, though the proficient instrumentalist edge can help with that. It makes instruments perform better when using a repertoire they are not designed for. I.e. a tragic instrument, with the edge it would perform better when using any of the other reps. The edge does not help when using an instrument designed for that repertoire though. So that edge does nothing when using a tragic instrument to sing in the tragic repertoire. It basically negates the need to carry an instrument for each rep. You can use a romantic instrument for comedic (which might be nice if you can find an instrument that progs but is not in your preferred rep) I am not positive that the edge will match up perfectly against an equal level instrument designed for that rep, but it certainly improves it.

On instruments, there are lots and lots of instruments out there, and some characters will probably tell you, or maybe give you, a decent instrument for any repertoire. Generally try to have one that is at least level 35, as you can almost always find one of these. Generally lots of people like to ask other bards about this (if the bards they ask are not evil), or heralds, or fortress folk. I know I have helped lots of new bards with instruments IC. Since only bards use them, there always tends to be decent ones available for any repertoire.

It can be tough for a new bard to get the hang of instrument knowledge, depending on how much game knowledge you have, because not all the instruments are things you would see very often, or be looking for if you are not playing a bard. Legendary awareness may help as well, as what looks like a common instrument might be a very nice one, but you may not know it.

The trick with instruments is to look at them, and most of the time you can get an idea of what they are used for.
Drums/horns tend to be epic for example.
String instruments tend to be romantic or tragic.
Comedic there tends to be a few less of, but I never used that repertoire enough to bother learning all the locations. But think woodwinds and other light airy instruments.

I won't post specific instruments of course, but you can check in places where you might expect. Music guild halls and inns might have people with them, even some shops sell decent instruments.