Keeping a shy dog “friendly”

Help. My dog is a pound rescue. She is a Pit type mutt about 3 ish and I’ve had her just over 2 years (she was somewhere about 6mo-1yr when adopted). She has always been a little shy around unfamiliar people and situations thought friendly especially if she thought food might be involved, but she really blossomed when we did a Petco “training” class and was able to meet and greet new people and appropriately meet new dogs, and strangers (say the electrician for example) could come and go from the house without a peep from her as long as I greeted them. Since then we have had a few bad experiences that have made my shy dog insecure. First we were doing a training session in public when a strange man came up behind me, scared the crap out of me, and since then she barks at strangers if she deems them too close. Then we have had a few run ins with unruly loose dogs while out for walks. She does nothing more than stand at alert in any of these situations and bark. But her bark is pretty ferocious sounding. With CoVid it’s been hard to keep our socialization up as well as I’d like. We go run on the local trail a few days a week, so we see tons of people and dogs (mostly well behaved) but everyone keeps a distance so she is very comfortable with that. What I want to be able to get back to is meeting new people and dogs in appropriate situations and to handle it quietly and well. Bonus if I could get her to happily greet new people.

P.S. I do appreciate that she is alert and protective if the situation would arise, but I’m afraid she will become reactive if I don’t start working to fix this now

If you could find a dog training group that was meeting outside, that seems like it would be helpful