I have a nightmare neighbour too, so you're not alone!
I had a lovely old man that lived next to me when I first moved in, he would help me out any time I needed it and would even invite me in for tea etc. He had been there for over 10 years then suddenly just as the pandemic hit, he left without even saying goodbye!
In his place was a horrible misogynist! I bumped into him one time and casually asked what happened to the guy that used to live there and he seemed fine, told me they did a mutual swap and I thought he wasnt that bad, then one time his post came to my house and I opened it not reading the name and noticed it wasnt mine when I was some car insurance letter! I felt so bad i wrote a note, apologising for opening his mail and posted it through his letterbox and thats where the issues started!
He randomly started taking in my amazon packages and since amazon didnt notify me they delivered it to him, he came to my door really aggressive shouting at my partner at the time and I and throwing the package at us!
This happened more than once, on the last occasion my ex opened the door and he threw it on the floor and demanded my ex pick it up! Such anger and disgust in his voice I came out asking what's going on and the man was shouting how we always order stuff but are never in to get it! I then told him not to accept any of our stuff, its as simple as rejecting it and we never have to see each other!
Even after this, I wrote a note on amazon saying never deliver to his house and I still caught him taking my package in (or my tapo doorbell) when amazon never read my note! This time the guy stole it! He made false reports to our housing manager about our dog and has just been really aggressive!
Iv never known anyone like him! Its been years since all this happened, I now have a 2 year old and he thankfully gives me the silent treatment with occasional dirty looks! He is such a nasty piece of work and was like that pretty much the moment he moved in! I'd love to move but im in council housing and they have no intentions in helping me out even though my son and I have to share a bed the place is so small!
So I can understand just upping anf leaving isnt always an option.