The issue with Thievery is that it's too simple that you can't add anyway to deal with it.
Let's say that they become more aware and become harder to steal statwise (they need to have higher wits), that won't stop me from using my worst thief first (Low Finesse, thievery from gears only) before stealing again with a master thief (Very high Thievery and Finesse). Logic-wise, they're not going to suddenly become super aware that they can prevent an invisible master thief from pick pocketing them.
Here's a way to deal with repeated thieving. Let's just say add a cumulative penalty of -25% (or -33%) for weight and gold value for each theft after the first. So if you can steal 100 and 100 the first time, then 67/67 the second time, 33/33 the third time, and then add it would be 0/0 the fourth time, but let's boost it up to 10/10.
That would be a way to limit the amount of thieving you can do without nerfing it into the ground for one-thief-only parties. If you want to make 4-thief parties, then go ahead.
