Since you mention it, I think an even better system would be to simply have every crop remove all three nutrients, but in different amounts. Crops need all three to grow, of course, and a host of minerals, as well as a certain range of pH, but to keep things relatively simple, focusing just on NPK is more than enough to improve realism and add challenge to the game. Most crops will remove more N than P or K--even legumes, if the aboveground leaves are harvested or not incorporated back into the soil. This website has a handy illustration of some removal rates for different crops when completely harvested: https://www.ipni.net/app/calculator/home
If you click around a bit, you'll see that most crops listed will remove N the most, K the second most, and P the least of the three. There are some exceptions when it comes to the P and K side, like sunflower grain will take more P than K off a field, but in-game they're an N--which should be the case for nearly every plant, if we had to pick one of the three to focus on. That's why I think having each plant take NPK (unless it's used as a cover crop and/or is a legume) but in differing amounts would be the most realistic and add richness to farm management in-game.