I find that the trade difficulty in the game is unrealistically hard.

Based on the real life trade, I tried to re-create the James Harden trade in game. I was only able to get the players, and not a single pick (whereas Houston got 4 picks and 4 swaps in real life).

Is there any way to lower the trade difficulty? I know this is an extreme example, but it has been something I've noticed throughout my time playing the game

There is not but I think that's with good reason. Obviously the same conditions don't exist in the game (there are no pick swaps, only 5 players/picks per side and trades can only go 3 years out) but if someone was able to trade a star player and end up with another former all-star plus all those picks and swaps people would complain the AI sucks and its way too easy to take advantage of it. I'm glad that the trade AI is realistically difficult because it sets the game apart from some other basketball games where you can easily take advantage of the AI.

There is a solution for you though - if you swap your control to commissioner control in the upper right corner and then go to the trade page you can force through any trade you want.