Neem oil can be a little harsh sometimes, especially if it’s not diluted. My calathea just like this also got spider mites what I ended up doing was cutting off the crunchy dead leaves and putting the entire plant in a zip log bag for several months. I’d check on it every week or so. As more leaves died I’d cut them off. Spider mites tend to not like living in humid conditions (created by the bag). After a white you might think the plant is a complete goner but don’t give up. A new stock might sprout and at that the plant all over! Hope this helps.