1. As your fish go up, perfect fish cost the most, but they don't increase in value.
2. The more books you write, the royalties do increase, however, I have level 10 in writing and one book will be worth a lot of money and maybe the next book he writes in the genre will be less, not sure why, but maybe random. If they have finished all of the challenges and reach level 10, you can make a fortune writing books, but it takes time.
3. Sometimes the same painting will not always be a masterpiece, but again, when they reach level 10, most paintings are at least brilliant. I have had a painter that every portrait they paint, are a masterpiece and some painters, whether they have all challenges met, seem to hardly get a masterpiece. Not sure if you can actually tell if a painting will be a masterpiece, but something that I have tried and it has worked, is have a room that is scarcely decorated. Put an easel in the room, when your painter starts painting and they get a moodlet of "beautifully decorated", that usually means the painting is a masterpiece or at least brilliant. I did test this out and it seems to be true for sculptures also.