Your thumb is not-so-green because you have a few basic misunderstandings about indoor plant care.
Yellow leaves are a generic symptom and can be caused by under or over watering or too little light or damaged roots.
Root rot is not usually a disease that warrants disinfecting the pot and replacing the soil. Root rot is most commonly caused by keeping the soil too moist and not allowing sufficient oxygen to penetrate into the root zone. The antidote for root rot is to allow the soil to dry out deeper into the pot.
Replacing soil is very traumatic to plants as it easily damages most of the tiny root hairs that do most of the work.
Getting your ZZ back on its feet will be difficult and require considerable patience on your part. It should be potted in the smallest pot the roots will fit into along with just enough porous potting mix to cover the roots. Keep it in a warm, moderately sunny location. Try to keep the potting mix just barely damp. Expect additional leaf yellowing, so be patient while the roots and plant try to recover from the trauma they have experienced.
For the future, don't make any assumptions about the cause of yellow leaves. Always keep your new plants in their original plastic nursery pots and leave the soil and roots undisturbed.
