That sounds incredibly tough and very isolating for you. I think it sounds well beyond the ‘normal’ stranger anxiety.
I think, in your shoes, I would try and very gradually desensitise her. Maybe start with one ‘extra’ person and try and gradually and gently increase her tolerance. Play with her with toys she really likes in a ‘safe’ space that you can control and have the person do something else that is interesting just in the periphery of her vision. Let them get gradually closer until the point where your daughter looks uncomfortable but not upset then have them move away again. Tell them not to look at her. The initial aim would be to get her progressively more comfortable with one person in her space without the pressure of that person looking at or speaking to her. Once she’s more comfortable with them being around you can gently build up to them playing next to her and maybe handing her a toy etc. It might take weeks to widen her ‘circle of trust’ to one extra person, but once you have you can build on it. It’ll also give you a bit of breathing space.
I know it’s really hard, but I would imagine that you probably get quite stressed or upset now by people approaching and her reaction. Try and stay calm and ‘light’, because she’ll pick up on your stress and associate it with people, causing a bit of a vicious circle.
I would consider visiting the GP, and taking videos with you to illustrate the extent of the issue. Did she have her 12 month development check with the health visitor? Could you speak to them? With both health visitor and GP, it really depends on how good your practice and area are on whether you get any support. Is she starting to talk and meeting other milestones?
When you are out with her, is she in a pushchair? Could you have her parent facing (if she isn’t already) and maybe even put the hood down and rain cover on so she feels ‘cocooned’ and safer. World facing pushchairs can feel very exposed to some children.