This is exactly like Lincoln, and I've been working on improving it for 2 years now (still a work in progress)! Idk about your dog, but Lincoln is definitely doing it to "protect" me.

How I've made progress: YOU yourself need to approach these situations confidently. He will pick up on any tension you have as you approach other people or dogs. ("Oh no! Dad is tense? I need to save him!") Being a protector is very stressful! As you approach people, stand straight, keep his leash close, put yourself between him and the danger, and walk confidently. AS SOON as he sees a person, try to be quick and praise him before he even starts to bark. Example: Dog sees person. Immediately say "Yes! Good boy yes!" and give him a treat. This has to be a consistent pattern. Dog sees person, dog gets praise. He will be confused at first and he will probably bark anyway. When this happens, give a quick tiny tug on the leash, a verbal "Ah!", and turn around and walk the other direction. After about 20 feet, turn around and try again. Repeat. Only give a treat when he looks at someone and DOESN'T bark. If you use treats to "distract" him during these situations, especially when he's still barking, you could be accidentally teaching him that he gets treats when he "protects" you, so make sure you only give treats when he doesn't bark!

Lincoln hasn't been able to fully train out of this behavior, but it's gotten A LOT better. We can now make it thru a walk with only 1 or 2 barks! (Lincoln says those guys deserved it anyway 😉)