Unrelenting barking

Hello all, I have a 2 year old pure bred Shetland sheepdog. I have had her since she was 3 months and she has always been a very fearful dog. Since she has been about 8 months she has gotten into this horrible habit of barking at everything. I have looked through multiple resources and tried multiple things, but cannot seem to make any headway.

She is a very vocal dog in general... It is her go to for any emotion... Fear excitement etc

The real problem arises out of fear I believe. Whenever we come home, until we enter the door and she sees it is us we hear very shrill panicked barking with no break. Whenever she sees a stranger outside or hears a strange dog bark she starts to bark aggressively. This only occurs in our house and yard. When she is out on walks, she will just cower at my legs if she sees some other person or dog. She also barks out of excitement such as when she knows she is about to be fed... These are much happier barks but I figured I would mention them for completeness.

Now to talk about what we have tried... Originally we would just discipline her... Tell her no or get out or scold... She got the picture that she wasn't supposed to bark but seemingly couldn't help herself... She would bark and then immediately run away from us fearing the negative reinforcement. Then I went to attempting comforting, I would sit with her at the open door and softly stroke her and tell her it's okay and try to keep her calm so she would start to connect seeing these things and being safe... The behavior lessened in severity while I was right next to her, but she would still bark and then the second I stepped away it was right back to her old ways... I have also tried distraction... Give her a sit command or shake ...something she knows and then give her a reward to try and distract her from the fear... She will execute the command but refuses to keep eye contact and will just nervously keep shifting her glance Lastly I tried just ignoring it seeing if she will calm herself down and begin to realize things are okay... She would just go nuts the point where I feared for her health. She would literally bark non stop if someone was working in a nearby yard until they were out of site... If they were staying for long periods it was unending... She would start panting from so much barking.

I am really get frustrated with it and do not know what to do.... I feel like she will have a heart attack out of fear and I do not know how to help with the anxiety that I believe is the underlying cause of her barking...any help would be greatly appreciated