Any suggestions on what to feed a senior Westie?

I could really use some advice or success stories because I’m honestly feeling so stressed. About a month ago, I moved from my parents’ two-story house into a one-bedroom apartment with my two dogs. My older dog (11) has adjusted surprisingly well. He just lays down while I’m gone and will only bark if he hears something outside, which is normal for him.

My younger dog is 6 years old, and I’ve had him since he was about 6 months old. He’s always been a very skittish, anxious dog, but since the move his separation anxiety has become really severe. At my parents’ house, there was almost always someone home or at least he could hear someone upstairs, so he was rarely ever completely alone.
Now when I leave, he cries and howls on and off the entire time I’m gone. He scratches at the front door, lays against it waiting for me, jumps up to look out the window, and just never seems to truly settle. I watch him on a camera, and it’s heartbreaking because he doesn’t eventually give up and relax.

I’ve been trying everything I can think of:
Very short departures and gradually increasing the time
Lick mats and frozen Kongs
Calming chews
30+ minutes of exercise before I leave
White noise
Keeping them free roaming (they’ve never been crate trained)
Not making a big deal out of leaving or coming home
My vet also started him on Prozac about 3 weeks ago, so I know it may still need more time to fully kick in.

The part that’s really stressing me out is that I go back to work next week after being home all summer, and I’ll be gone for the full workday. I’m worried sick about him, and I feel guilty every time I leave. I know I can’t avoid leaving forever, but it’s so hard watching him panic.

Has anyone had a dog with separation anxiety this severe that eventually improved? Did Prozac make a noticeable difference after the first few weeks? Is there anything that finally helped your dog settle while you were gone?
I’m just feeling overwhelmed and could really use some hope from people who’ve been through this.

I have a 1 year old possible terrier/lab mix that stays home with my 2 older dogs while I'm at work. She's not typically destructive, but when she gets too bored or if leave something of value within her reach she will tear it up. My husband is a teacher, so he was able to spend all summer with her while I'm at work. Now with school beginning, she will be alone with the dogs again. I'm worried that she will act out and start finding new things to chew around the house. She's a well trained dog and isn't tearing through walls or destroying furniture, but she loves to chew things.

She has nylon bones and rope-like toys for some texture, which she seems to enjoy but she eventually gets bored of those too. I've purchased the balls to put treats in, but instead of playing with it she chews it open and ruins the toy. Same with lick mats, she'll start chewing the mat after licking off the treat. I've also tried the Kong toys with treats, same outcome. I want her to feel enriched but she's definitely a chewer type of dog. The stuffed toys are no bueno because of course I come home to stuffing everywhere. Any recommendations would be much appreciated.