Help with substrate

My parents have an aquarium that they want rescaped & they asked me to do it since I have experience with aquascaping. Im trying to figure out what substrate would be best for them so I need your help/advice. They want live plants in the setup & I can’t decide what’s best so what option would everyone suggest? Option 1 - keep their old substrate (river rock) that already has 15+ yrs of bacteria & nutrients or Option 2 - use organic potting soil topped with sand? The back & forth im doing is…well option 1 already has beneficial bacteria & nutrients & fish can be added right away, but long term option 2 might be best but they’ll have to wait weeks if not months to add fish due to the high amount of ammonia at the start. What would you do Reddit? Help me decide plz 😁