Extreme separation anxiety almost 5 month old puppy

I have an almost 5 month old shih tzu puppy that seems to be suffering from extreme separation anxiety.

This anxiety started from when we got him (9 weeks old) and has not improved no matter what we do.

Things we’ve tried - Flitting (leaving the room for 30secs then immediately returning) and increasing the amount of time gradually. He manages for about 1 minute but then starts screaming and whining - Crate training - he will walk in and out of his crate, eat in his crate (with door open) but the minute you close the door he will abandon whatever he’s doing to scratch, scream and bang himself against it. It does not matter if I’m right beside it with my fingers in the door. - putting him in a playpen - similar to the crate if we move even slightly away from the playpen he will jump, thrash, scream and scratch at the sides of the playpen - leaving him in a room with a window so he can see us leave - he will watch us go and the immediately star barking, pawing and screaming at the window until we return. - exhausting him - no matter how exhausted he is, if we leave he immediately wakes and moves so that he is close to us. He does not settle when we are not in the room - Snuffle mats, high value treats, Kong, chews. He loves all of them and we only give them when we are leaving but the moment we go, he abandons it (does not eat or touch them) until we return. Instead he screams, barks, thrashes etc. the moment we are home he immediately goes to grab the treats from the various toys - desensitisation- we put our clothes and shoes on, rattle our keys and then sit on the couch. We don’t fuss when we leave and don’t fuss when we return.

Yesterday, on the advice of our vet, we left him for 1 hour in a larger space (our kitchen/laundry). The vet said to go for the full hour and then come back (ignore his crying and he would stop eventually).

Before we left, we took him for a walk which incorporated free running and playing with other dogs, then we came home and played with him, then did some training and made sure he was fed and had water. We then put the Kong, snuffle mats etc down and left.

I had a camera on so I could monitor him and the moment we left, he abandoned everything and paced, panted, screamed for the entire hour until we got home. He did not settle for a single minute.

I’ve been told to just ‘persist’ but I wonder if anyone here has any advice, I’ve had dogs before and this is the first time I’ve ever dealt with something like this