Looking at your plant, and based on what you said already..it may have been overwatered before, and it seems the color of the leaves is too deep green too. This tells me the plant was in a very shaded area. Although it can tolerate that, it will add more vigor to the plant to get more light. It may also be adjusting to the seasonal change, sometimes as the light lengthens and temperatures improve with the seasonal transition, the plant will naturally drop the older leaves below. As long as the stems and branches are firm to your touch, I would not be too worried. It is just a phase most succulents do, dropping the older leaves and redirecting their energy to newer growth either at the center of a leaf cluster or could be up and down the stem or branches.
I do not recommend adding sand to the soil, since it later clumps up and makes it hard, better to add perlite or pumice to keep the mix airy and well draining.
Typically, water less and sparingly when conditions are cold or cooler and water more when it is warm and dry. That is what I try to remember, especially if my succulents are outdoors. Indoors, it takes much longer for the soil below to dry up. Better to water once with water draining out of the pot, then leave it alone for awhile, feel the soil after a week or two. I usually put a big rock inside the container to help me determine if the soil below is still damp. If I remove the rock and it shows damp soil, then I don't water. If it looks dry, then okay to water. Crassula ovata does not have a big root ball, so you really have to be careful with watering. It is quite excellent in conserving its water through the stem and leaves.
Your plant still has a fighting chance. Lots of green..lots of hope!
You can also get some good cuttings as suggested as a back-up. Let them callus and dry, lay the leaf cuttings on top of the soil, and it will either form new roots or leaves first.