I don't agree with the idea of your dog being PTS at all. I'm saddened and stunned.
I completely understand what it's like to feel overwhelmed by all the different techniques and advice out there. It can cause you to fall into periods of inaction because you don't want to sink months of time into the wrong technique or make things even worse. Then you end up feeling like your whole life is training, because it takes up so much space in your head. But without actually making clear progress.
Your dog sounds exactly like mine. I adopted him at 8 months, and he's 16 months now. It's not always down to bad breeding. Mine is purebred, health tested parents, champion lineage, yadda yadda, from a good reputable breeder. He still has the exact same issues that yours has. Don't go thinking that your dog is this badly bred, messed up creature that you can't shape. If you do, you won't feel equipped to change anything.
Focus on the one thing that will make the most positive immediate change to your life. That's the separation anxiety! My dog barked the place down the very first time we left him - but now he can be left for 4-5 hours, no problem. This can be solved!
Do you ever feel like you're maybe behaving awkwardly, anxiously, a bit robotic around her, when practising door/leaving stuff? I know I did, when I was following all of these individual microscopic steps. So if it helps - as soon as I started just being CASUAL, things started to change. I did the door stuff, built the time up, and if he barked, I just waited for a millisecond of quiet before entering. Does that make sense?
With nervous dogs, you want to think about her life in the general, global sense. Not just individual triggers and issues, but ensuring that she can find calm in her day. Anything highly stimulating, like doggy day care or even one-to-one play dates, create stress because they're such "fizzy" events.
Do you understand trigger stacking and cortisol? Because stress remains in the body for around 3 days, you basically need to space out "big exciting events". For instance, if she's had a playdate, or snapped/lunged/reactively barked, try to give her calm for a few days. Don't freak out if she gets re-triggered within that time. Just do what you can. Whenever my dog has a playdate with a dog friend, I do no other social things with him for at least a few days. Whenever he's had a reactive outburst, I avoid playdates for a few days. That kind of thing. I just give him time to recover.
I also find it helpful to have set times each day to help me remember to work with my dog. Otherwise I forget and get sidetracked (by Netflix...). For instance, every afternoon, I work on training with him. Two minutes, or 30 minutes. Doesn't matter. Every evening, I spend an hour reading training books. Between exercise, games, and training, I easily spend 2-3 hours a day completely on my dog (or things related to my dog), on an average day. I'd say that's a realistic commitment to make.
Meanwhile, can you look for an alternative to day care? A walker who offers solo walks, or a sitter/boarder? She needs a calmer, more one-to-one environment. Many dog carers are totally happy to go slowly with nervous dogs and do a few initial meetings to build trust. Half of the dogs I walk were terrified of me when they met me, but are my best buddies now. Daycares are just too hectic to focus on each individual dog properly.