I also like the GoPro idea. And if you can, don't place it in an obvious spot. When you use your phone to record, people know right away you're recording. Most people will change their behavior when they know they're being recorded. So if you can, place it in an area on your body or on the dog where it's not so obvious, then the chance of you catching something iincreases. It's sad that we live in a time now where we need dashcams in cars and GoPros on dogs to protect ourselves in case of lawsuits.
I use something called NextDoor ap on my phone that connects people in local neighborhoods. Anyway, there was a posting last week about neighbors complaining about a neighbor's two dogs barking all the time. (A lot of dog related complaints...including people not picking up after their dogs, etc.) They were talking about whether they should file a complaint to the authorities about it. I asked them all if any one of them actually talked to the offender face to face instead of immediately filing a complaint. No one's talked to them in person yet. I told them they should be neighborly and give the person a chance to correct the problem by either sending the dogs to daycare or training or whatever. Anyway I won't bore you with the rest of the story but all these problems are making me contemplate getting a GoPro myself to protect my dog and I in case I encounter some problem neighbor. (I've talked to other dog owners around me over the years and there have been litigious-happy neighbors.) Well, I figure I've got dashcams in my car and cams in my house, why not one for the dog? Meanwhile I can also get some good clips for the dog's IG page.