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.
The thread is simply an executable class, the ScheduledThreadPoolExecutor handles the management of it. How ? Who cares, it was optimized for you by some high-grade dudes years ago and added directly in Java.
Seeing how much tasks are generated by seconds (network, AI, and random tasks), it probably costs almost none CPU/RAM. You only have to assure yourself to avoid to bottleneck the system.
Not sure why you need this answer, your best bet is to make tests yourself because litteraly none did that before you (at least, I didn't).