the thing is that earning money by painting largely depends on luck. Sometimes you make a lot of valuable masterpieces back to back and then other times you make a lot of cheap ones. with the mixology method you described you earn the same amount every time, so it'd be more consistent.
However, if you have city living, using the wall to sell any expensive masterpieces you make at 300% markup can make painting extremely profitable.
if you have get together, some people swear by the infamous "painting clubs" where you get 8 sims together in a club, set the activity to paint, and take all the paintings for yourself to sell... I haven't really been sold on it, it seems slower than the methods I use when I want to make a sim rich quickly without cheating.
personally, the fastest way I've found to make money is by selling virtuoso violins at 100% markup in a retail store. Doing that I could make anywhere from 200-300k in a day of selling when the store is brand new, without any perks, and by the time the store has some good perks it's possible to make 500k-1m in a sim day if I try hard enough. The only thing is that it's much more involved than painting is, because you really have to interact with the customers to get them to buy them.
