My dog is like yours op. People suggesting coaxing with treats and sending her to daycare have obviously never worked with a truly anxious dog. A dog having a panic attack will not take treats. If I sent my dog to daycare I don't think he would ever recover, and it would be very dangerous as a terrified dog is a potentially aggressive one.
We are also seeing a veterinary behaviourist. They gave us a "sink" analogy and explained how once the sink is full he is over threshold and will react. The hot tap is things he is afraid of; the cold tap is things he likes but are stimulating (eg running around, chasing balls, playing tug, obedience work). Both taps fill the sink. Our focus is draining the sink and that means reducing cold tap (fun) stimulus too. For the time being he stays on lead and is only offered low arousal activities. Once the sink is consistently less full, he will be able to learn new things and we can gradually add more stimulus/fun/training.
At first I was very sceptical as dogs NEED to blow off steam right? Wrong! He is so so much happier with less exercise, less activity, less stimulation. (And he is a malinois cross!!) 2 x 30 min sniffy walks/day and some freework. That's it. The rest of the time he sleeps. When he was having 2hrs exercise/day and lots of play time, he was a whining, pacing, anxious, barking, reactive mess.
See what your behaviourist thinks, but it sounds like your girl is not benefitting from outside walks at the moment. It's too stressful and then her sink will be overflowing for the rest of the day causing undesired behaviours at home. I would cut out walks completely and focus on calm enrichment inside - my dog loves freework (great for confidence too) and destruction boxes. Lots of things to lick and chew as that's very calming. Just take all the pressure off for a few weeks and she how she is then.
I really sympathise with you because it's so very hard and the grief is real. But there is hope: my dog is dog aggressive as well as being terrified of everything and I have even thought of pts (more than once!). But after 6 months of drainage he is coming on great - he can take treats outside now and that means he can start learning.
I did move house though as we were in the town centre and it was too much. Honestly, I can't imagine my guy coping in central London.
So sorry for you and your pup x