Separation anxiety

Hi all, we have a 1 year old show line cocker spaniel who is attached to just me. We have a gate at the bottom of the stairs as he’s not trusted alone in the bedrooms. When I go upstairs without him he will bark and whine a lot until I come back down. He’s fine when anyone else is apart from him it’s just me. We are going to start small training sessions where I go upstairs my partner sits and rewards calm behaviour but I was wondering if anyone had any other ideas that helped them!

Below are a few things I think are note worthy or give insight to situations in which he’s ok.

1. He sleeps in a crate every night in our bedroom. No fuss or whining until about 6am on workdays, 7am on weekends which is when we usually get up to let him out.

2. He comes to work with me at a daycare and is not bothered when we are apart or when he’s alone, no whining or barking.

3. If I go for a shower he’s happy to sit outside the shower door, doesn’t need to see me won’t scratch on door or bark.

4. He is reactive to cars, new people and dogs, our dog trainer said he’s an insecure dog. Not sure if this is related but thought it was worth mentioning.

5. I do 90% of his care, feeding, letting out for toileting, training, I use to walk him by myself daily but he’s currently not allowed on normal lead walks (per his trainer) so I take him out at lunch during the work day, he comes to work with me, I usually put him to bed, I give most of his treats.

Let me know if think there’s anything we could do other than what we have planned. Also happy to take help for his reactivity if you have any! TIA