I'm so sorry about the loss of your beloved dog in such tragic circumstances.
We went through this 4 years ago and were badly let down by our vets. They spent weeks telling us our dog was fine, when we knew he wasn't. In the end, it was too late and they couldn't save him. This happened during a covid lockdown and we couldn't even be with him when he was PTS. He was a large, robust, healthy 7 year old.
The grief, despair and anger we felt was soul destroying so I completely empathise with you.
After a few weeks, we wrote a long letter of complaint to our vets and the BVA. This was, in some way, cathartic and they did, eventually, acknowledge his cause of death. We also changed our vet (we also have cats, and we registered them with a different practice in another town).
I know somebody is going to come along and kick me in the head for saying all this, that they'll tell me it isn't helpful or appropriate... But they need to understand that when you have put your trust in the vets and they let you down, it is a different sort of loss. I want you to know that others have experienced this and been through the grief being tinged by rage and frustration, and the wish that things had been done differently.
Honestly, you will get through it, but it will take a long time, and there will be moments when the anger and frustration will resurface. Do not blame yourself; there is probably nothing you could have done, it sounds as if the decisions were taken out of your hands, as was the situation with us.
I'm so sorry that you're going through this horrendous experience. Be gentle with yourself.