Thinking more like woodcutting and then planking. Least that's what I'm working on doing now.Stop trying thieving. You will find there a lot of PAIN and useless hard work + non automated tasks or leveling on some stages and it levels up slow. In addition to all that, you will have to level up your agility as well as thieving and so on. Also there are many other things that make it hard even when you try to do it with just one bot.
Fishing is good only on very high levels.
As it has been said before, Woodcutting is solid. Yell trees have high price and it's fully automated. You just need to buy a cheap axe once.