Separation anxiety training question

Hi - I adopted a 1 year old rescue last year and quickly figured out that he has really bad separation anxiety. I started working with a trainer about a month ago who is certified in the "be right back" style desensitization training, where you very slowly desensitize the dog to departure cues and then slowly extend the amount of time you actually leave the dog alone. I am seeing progress, but it is SO SLOW.
My trainer only wants me to train once a day, 5 days a week. Just wondering if there are any other points of view on this. I have time to train maybe 2x a day on some days, and taking 2 days off seems like a lot. At the rate we are going, I will be able to leave for 15 minutes by April, if I'm lucky.
FYI - I just want to say I tried other methods before this, and my dog just completely panics if he's left alone. A fun chewy, a cute toy, a t-shirt and a crate make 0 difference. I definitely believe that this is a panic disorder that requires a really slow patient approach and possibly medication, I'm just wondering if it needs to be THIS slow or if others have had faster success with more frequent training sessions.