Wow! You really did start KEM farming early @frosted1414, I don't think I would have been doing it at level 16 myself, but then again it hadn't been invented way back then as we had to wait around for them to release a new level with new stuff every few weeks.
I think @4junk3000 has a valid point there as well, he's kinda saying don't rush it, enjoy the process so you don't run out of stuff to do too soon, and end up like us old hands who have nothing to do in between events except sit on here either moaning or trying to help out others.
I seem to remember the first 63 levels being fairly busy with storyline quests and such, after which point it's just level, level, level till you hit 939, so I'd probably recommend that as a decent time to start farming for donuts and increasing that bonus.
@1EllieG7294 's tips are all good ones but I'd add in the yearbook (don't know what level that appears at) as a decent source of bonus increasing items. There's a handy breakdown of all the items in it and their % value over on the wikisimpsons and if you keep refreshing to mix them up till you have a whole row of high % items then that can be a great way to massage your bonus % without getting overrun with too many multiples of the same thing.
The best advice really though is to enjoy the game, don't let it become work and take it at whatever pace you're comfortable at, despite the regular rumours to the contrary I don't believe it's going away any time soon.