Since you already have an established relationship with a behaviorist you know and trust (and who knows both Apollo and Roxy), it might be worth asking them what they think.

At the risk of complicating things even further, I think it's important to think about how a new furry family member might (or might not) affect Roxy and her anxiety. It could be worth mentioning at your next follow-up appointment. I know that some behaviorists will even help their clients with the selection process, evaluating the potential new dog's temperament to see if it is a good match for the family's specific needs. I'm not sure if your behaviorist provides this particular service, but if they do...