What you can try to do is to change dynamically each executor's thread size based on your current cpu usage (ThreadPoolExecutor#setCorePoolSize). Although, i don't think that you will need such an optimization.
Let me en light you since i had the most clients in this forum. The majority of them will waste your time. 30-40% of your client will find excuses such as "I just got Covid", "i haven't receive my payment yet", "i need buy my wife first in few days we talk". People like this mean they will never buy your product. They simply either found something else or they didn't mean to pay that much.
So what you do. Avoid big conversations, be detailed but don't spend hours talking to 1 person, he will drain all your energy. Do not work without any payment.