So sorry you are going through this.
I had a similar experience with my birds... I lost about 40 birds and was left with only 8.
They stopped eating, stopped doing anything, stood in one place, had water in their crops and were vomiting the slimy water. They got paralysis and died within 24 hours of showing symptoms.
At the same time I also lost all my chicken flock of over 50 fancy bantam breeds, and several of my parakeets.
I gave them all different antibiotics and other treatments. No help. Some were isolated, some given heat, nothing helped at all.
Only way you will know for sure about the disease is if you send birds for autopsy. But, most birds carry all the more common virus and bacteria anyway and don't show symptoms until they are stressed by something, like the wet weather or a retro virus.
In my case I found out it was an 'officially' an outbreak of bird cholera. All over our region farms lost thousands of ducks and chickens, and dead birds of other species were found dead. But I suspected the real disease was an outbreak of a strain of avian flu, but the authorities did not want to let people know about that. I doubted the official explanation as my pigeon were confined to the coop and covered aviary and had no access to any water outside, and the coop was 100 percent rodent proof too, but mosquitos could get in.
All my birds were in perfect health before, and all were in roomy clean pens... and the chickens were free range and roosted in trees, so I know I did nothing wrong. I nearly gave up the hobby then after that traumatic experience.
I would advise you to move you healthy birds to a new coop.... which is dry and not overcrowded. Change the feed in case its contaminated, and give them all an antibiotic for a week or so. I would say get some to a vet, but I know avian vest are hard to find and super expensive!!!
Wish you good luck.