Welcome!
AI3 is a fantastic first target, because it's a great place to see some coherent strategies in AI decks while still being relatively easy games. Once you've learned what there is to learn from AI3 (which sounds like right about where you are) you have some options on where to go next for AI targets. Even still, though, AI3 remains an excellent target, as it is the steadiest electrum farm available in the early part of the game.
AI4 (halfbloods) is an excellent place to go for money and it is well within reach. Note that it is rather inconsistent with profit. It's profitable because there is a roughly one-in-twelve chance per win that you will win an upgraded card which you can then sell for a bit under 1200 electrum. Because of this, it is quite profitable over time even if you don't have anywhere near as high of a win rate as you would in AI3.
Arena (specifically bronze league) is another great target. It's less profitable in terms of electrum than AI4 and approximately on par with AI3 (assuming you sell all cards you win, which you shouldn't), but it is a fantastic source of rares, including shards. It's also a great place to see a wide variety of player-made decks. If you want to build your rare collection, this is exactly where you should be. It's more difficult than AI3, but possibly slightly easier (or on par with) AI4.
You can also skip straight to false gods if you're so inclined. There's a fully unupped/rareless deck called firecell scavengers which can beat FGs with a low win rate. All cards won from FGs will be upgraded and sell for a lot as mentioned above. This may possibly be slightly more profitable than AI4, but it may not be (there hasn't yet been a full profit study on this deck). If you choose to go this route, realize that your win rate will be extremely low and that you might be at it a while before you start being profitable (so you'll need to have a bit of cash saved up in case it takes a while to start winning upped cards). Also, make sure to read the deck's thread, as it requires that you play it in a very specific way. I do not personally recommend this option (since it is extremely inconsistent, IMO boring, and every game plays basically exactly the same), but it is definitely an option.
TL;DR:
AI3 is great for steady money. AI4 (or FG, which I don't recommend) is great for more money than AI3, but is less consistent so it may take longer to show its profit. Bronze league is great for rares.