unpopular opinion (i think...?)
Unpopular opinion, but I actually think it makes perfect sense and is completely realistic for Cassie to have done OnlyFans. If you look at her character arc from the very beginning, she was heavily sexualized since she was just a young teenager. Because of her trauma and her father leaving, she learned early on that her value was tied entirely to male validation and how men viewed her body.
When she was in a financial bind or feeling desperate, selling her body was the only logical route she would know to take because that is exactly what she had been conditioned to believe she was good for. Cassie was always a people pleaser who got really good at hiding her mess behind a "perfect" facade, and her friends were usually so wrapped up in their own chaotic lives to look beneath the surface. I also really don't think she stole Nate on purpose to be malicious. Nate was simply the only guy who made her feel actually loved and desired, even if it was a toxic and twisted version of love. She just wanted someone to make her feel safe, and he filled that void.