Please remember that PTS is never a welfare issue. The dog doesn't have any concept of a shortened life but it is a hard,hard decision for you to make and I suspect that the guilt would be very hard to live with and that the resentment you'd feel towards your husband could fester. I volunteer for a breed rescue and have to say that the rehoming situation is absolutely dire right now even for relatively uncomplicated dogs, let alone those with any issues so trying to rehome through a decent rescue will be difficult - how would you do meets with potential new homes if it was a 'home to home' facilitated by a rescue? And kennel spaces are in short supply even if you think he would settle into one. Private rehomes are fraught with danger for the dog unless it's someone you know well and even then, dogs can and do get passed on.
Could you get your husband to agree to give it 6 more months with the agreement that you will PTS if the dog hasn't improved to an agreed (reasonable, realistic) level such as be able to stay in another room without losing his mind while you have visitors, being able to be left at home for very short periods and that you will work on finding some sort of dog daycare/home boarding so you could have days out or holiday for instance. For this to work, he'd have to agree that he would be supportive - or at least silent - during this process as the dog is probably picking up on the tension between you. Dogs do know when they are not liked and an already anxious dog will find this very scary.
And just to know that you had tried absolutely everything before PTS, I would change vets, medicate with fluoxetine and possibly trazodone added for a short period just to dial the anxiety right down. Keep his world tiny to start with - no visitors or new/stressful experiences as much as possible - accepting that his world may always be small and pick one goal at a time to work with such as a puppy pen beside your chair with a lickmat/kong/chew toy where you can still touch him, he can see you and look for him settling even a tiny bit so you can praise him. Make the pen a really, really good place, never punishment and accept that it will take time, will be 1 step forward, 2 steps backwards but there can be improvement in tiny increments but you have to be patient and persistent. Then you can work on getting out of the chair, returning immediately to walking out of sight for a split second and very gradually expand on that while praising any tiny bit of calm behaviour. You are working on a safe place where the dog can relax, without you and can use that for different situations in time.
Trazodone is only for short term use but could be useful for the initial period but fluoxetine can be given long term. My own dog has been on it for 5 years and will stay on it for life.
When we are having issues with our dogs, living with the dread of 'what next' we're tense and anxious too, which they feed from and we forget to look for the tiny bits of good behaviour and praise it. It's a vicious circle but as we're meant to be the grown ups and they're only animals, sometimes we need to take a deep breath, fake until we make it and reset everything. The meds aren't that expensive, particularly if you ask for a prescription to buy online but they also aren't a magic bullet, they allow the dog to relax enough to be able to learn - nobody, human or canine can learn anything useful when they are on the edge of hysteria, it's exhausting.
If after 6 months, you still feel that your, your families and the dogs quality of life is still poor, then PTS would be a valid option, just as it is now TBH but at least you could, hand on heart say to yourself that you had tried as hard as you could.