separation anxiety advice

I'm not sure if it's signs of separation anxiety or just a normal puppy phase. Basically I work from home and I'm away in the study door closed and my just turned 10 month old boy is perfectly fine with settling and minding his own and I can be in the study without a beep from him (unless he sees something outside) for a good hour or 2. But the second you leave the front door he barks like mad. I set up a camera the other day when I dropped someone off at the train station I was gone for about 15 minutes total and the full 15 minutes he was barking.

I'm assuming the difference is the fact he knows I'm still in the house when I'm working?

I've been doing the opening the door stepping out and immediately coming back in, ignoring him when we come back until we're settled in.

I was very conscious about him developing anxiety because I work from home which is why i set the boundary of him not being allowed in the study.

does anyone have any advice on how to get him to settle. I tried giving him a chew the other day and set up the camera and for the 3 minutes he was chewing he was fine then the barking started. I"m thinking of getting him a freeze ball which would last a lot longer than a few minutes. We've also got him a lavender teddy but he's not really interested in it.

My work contract ends on the 5th December so I'll have a few good week/ month or so to properly work on it with him before I start work again(which could potential not be fully WFH). leaving, coming back, building up the duration etc so just trying to gather as much advice as I can