Yeah, imo, compared to things like painting & book writing, or as was mentioned, mods for programmers, musicians seem REALLY limited in this regard. First, you can't even start writing a song till level 7 of an instrument; then you can't even LISCENSE said song till level 9; on top of that, writing a song seems to take much longer than painting a painting or writing a book; and FINALLY, once you do all that, you can still only license one song per instrument every 2 weeks. I haven't done the math exactly, maybe there's something I'm missing, like licensing songs makes a lot more money or something (but I don't think so). It seems extra weird when the musical genius aspiration has writing 4 songs as its 3rd tier requirement, which also has reaching level 6 as a requirement, so you'd think you'd be able to write songs at level 6? But no.
It all just seems extremely unbalanced, considering a dedicated painter can paint, like, 10 paintings a day and sell them each instantly for thousands of dollars, and its something I've wished they'd update in a patch or something. I suppose books & paintings can only be sold once, while songs you do hang onto (which...is kinda weird in terms of the book publishing if you think about it, but that's another topic), but that can be balanced easy enough just by slightly limiting how much the royalties can be (I've never seen royalties come in in the thousands, like paintings, for instance).
If I could redesign it, I think I'd make it so that
1: you can start writing songs at level 5 or 6;
2: you can license at that same level;
3: you're not limited by instrument, you can license as many songs as you have (tho of course, can't license the same song twice at once);
& 4: make the amount of time it takes to write a song vary by what kind of song it is, so simple ditties you can write early on take less time than epic ballads unlocked at later levels, with how much money you get from licensing connected to both the quality and how "high level" the song is.
These changes, I think, would make it much more on par with other creative skill crafting in the game, and also just make more sense intuitively for the player. Right now, in order to live off royalties, you really have to make your sim max out multiple instruments, which is not only a lot of extra work for the player, it also is really limiting story wise, if say, aesthetically you want you sim to really specialize in 1 instrument. I might not want my violinist to also play piano or guitar, and that should be a viable option in terms of living off songs & royalties.