Cages, even if they're kind of loose, can cause numb toes. Usually, numbness is from having the straps tight enough where they actually do you some good or it's because the soles of your shoes are too squishy/floppy. If you're going to keep spinning so frequently (might be good to have a rest day thrown in there if you can move some of your workouts to the weekend), you probably would like to get some cheap cycling shoes and spd cleats so you can use the other sides of the pedals. The stiffness of the soles plus the cleats helps you make circles instead of using your legs like pistons. All that pushing down can be overloading your quads and knees. Also, if you have the tension TOO light, you are lacking some stability the resistance gives you. This is particularly bad for your knees when you do standing exercises. You might also want to think about how your knees track relative to the centerline of the bike. Do you stick them outward? Far inward? Do you have any biomechanical issues with your feet, knees, or hips, that might impact your leg alignment on the bike? If you get cycling shoes, you might be able to correct any of those problems with wedges or good insoles.

Talk to your instructors and see if they can see if you're doing something wrong. It's hard for us to speculate over the internet, but we can provide you with some general rules of thumb.