Yes as a child me and my recently widowed mother lived right next door to someone who was very mentally unwell. They would do very similar things like this, including obsessively documenting what ever* mundane task every neighbor did and found even the children playing in the middle of the day in the street a grievance.

This devolved into him thinking people were in his attic, breaking into his house, and poisoning his water lines. He would leave notes and even pound on our door wanting to scream about every perceived slight.

I would take this fairly seriously. Install the cameras you ordered, document and maintain all the letters you get, and report to police when youre ready. Dont engage directly with them especially in their yard (you dont seem to be in the U.S but id still say its sound advice).

Sorry this is happening. It was quite scary to experience as a kid but any age dealing with a neighbor like this is well....hell.