What are these little brown/rust spots on the big ...

Hey there : you got both deficiencies at the same time , the little rust spots that's calcium deficiency. I also see yellowing between the veins of the leafs and some yellowing on the edges of the leafs too : that's Mg deficiency. Typical hydro problem when you dont have enough chelation agent, either add some chelation agent or if you want to keep your feed schedule, try a foliar spray 3ml/L three times a week, it should actually be enough for the problem to stop spreading and the plant to recover. Hope this helps ! 🚀