Answer: You have a real beauty with your Jade plant. Since it is a slow growing plant, the size of the trunk indicates it is an older plant. I understand your concern about keeping it healthy. Let's consider a few things:
Jade plants can tolerate being pot bound, since they do not grow very fast. Don't be in too much of a hurry to repot the plant frequently. However, if it's been several years, you may want to consider going just one size up with some fresh soil that is porous, containing sand or perlite for drainage.
Jade plants like it dry. Since you indicated you "may" be over watering it, that just may be your problem. They are better off under watered, rather than over watered. Allow the soil to feel just barely dry on top before watering. And, of course, don't ever allow the plant to sit in water. If you are unsure about watering, try a moisture meter which will help you determine the watering needs of individual plants.
Jade plants like a lot of sun. Bright indirect light is tolerable, but the plant will not have the healthy look that full sun will give it and its stems will be weaker. But chances are that the south facing window of your solarium may be just a little too "hot" for it. Try moving it back just a little so the intensity is not so great, or put it in a spot where it will receive some shade in the peak of the day.
Don't over fertilize your plant. Jade plants are slow growing, and therefore need less fertilizer than many other houseplants. Feed only once every 3 or 4 months.
Keep an eye out for mealy bugs, which are the most serious pest of Jade plants. Use a cotton swab dipped in alcohol to eradicate them. If scale insects infect the plant, just rub them off. Spraying could mark the succulent leaves.
Remember too that your plant is evidently quite old. No plant lives forever. Take some cutting from your plant and start some new jades, so that you will always have a lovely plant.
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