First time cat owner with extremely shy cat - need help!

I have had Olive (adult, female British shorthair mix) for about 2 months. I had to move apartments about two weeks ago, and that was scary for her. I adopted Olive from a local shelter - she had been picked up wandering along the side of a highway. She is EXTREMELY shy and skittish and spends much of her time hiding. I have been petting her as often as she wants to be petted (although I am still learning to read her body language and she does not set very strong boundaries so sometimes I unwittingly pet her for too long), and over the past week I have gotten her to play a bit (only for ~5 min. at a time, and halfheartedly at that). She is eating and eliminating regularly, has seen a vet and is more or less medically healthy. At the old apartment she lived with another cat who was mean to her, but now it is just her and me. She is definitely more comfortable than when I first brought her home, but not by much. She only lets me pet her when she is in her "safe zones," she has NEVER approached me to seek affection (or anything else, for that matter), she is scared to be out in the open (she needs to have her back to a wall and if I walk into a room where she is out in the open, she gets terrified and bolts off to a hiding place) and she is still reticent to play (and the little bit of play that does happen only happens in her "safe zones"). I have given her plenty of cozy hiding spaces in every room of the apartment, a scratching post in every room (which she uses) and a cat tree (which she does not use). To complicate things, she HATES to be picked up or restrained, and protests with much hissing and occasional violence, but though I respect her preference, I had to restrain her to give her medication for a gum infection for her first few weeks at home, and may have to do so again in the future (I do not have to pick her up for any reason presently). Should I be doing something differently? Should I stop approaching her and ignore her until she comes to me? I love her immensely and I just want her to feel safe and happy and loved, but this is my first cat and I am not sure what to do. Please help.

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