Did you call the police when he sounded the siren at night? Like 999? From what I read here there is enough there to get him cited if not arrested.
You need to keep a paper log of every incident. Keep it factual, just answering the w questions, no "dickhead was a dickhead again", just "Mr Smith turned on his radio at very loud volume at 6:30am on 6th December." "I asked Mr Smith about the loud noise on 6th December at 6:30am when I saw him outside in his driveway at 9:23am. He told me he did it on purpose."
I would call 999 for every noise complaint in the night and every threatening remark made against your brother at any time. You need a stronger paper trail for people to do something. You've complained to your council and you've talked to the police. It seems that wasn't enough.
Don't retaliate with loud music or deliberate harassment yourself. It might be hard not to. But you need to be spotless victims. You don't want a 'let's look at both sides' to give the impression you provoked anything at any time.
How is the third semi detached party feeling about this old guy? They must feel some of the nuisance? Surely they could corroborate that your brother isn't doing werewolf things at night? See if you could get them to write letters or call the police as well. The more neighbours join in, the better (but don't start a poster campaign, it should look "natural").
Is there something that can be done to stop foxes from coming into the gardens? If they are the "werewolves" here then this could be a way to root out that delusion.
Does the old guy have family? Is there a way to reach out to them? Not in "reign in your fucking dickhead father!" but "we are really worried about your father's mental state because he keeps making outlandish and unsubstantiated statements."
Can you google if there is a solicitor in your area that will maybe hear your case for no or a low fee to see if you can take this to court? Either the old dickhead or the council for not doing anything about him? That's where the paper log might be useful.
The only word of warning I would leave here is this: you said you're all not at your best. Taking legal action will be additionally stressful and will cost money and time. Another thing that would cost money and time is selling the house and moving somewhere else. In the interest of your mental health, you should probably discuss this first, which path to follow. And although it is cynical, you also need to factor in that this problem will resolve itself naturally, probably within the next decade.
Neighbourhood disputes are not great. Your home should be your castle and somebody is shooting cannonballs at it. I've seen one case in my parent's friend group that started with a fence encroaching on the other property, more than a decade of lawyers going back and forth, accusations of tyres being slashed, police calls for any minor infraction. This can consume people. Don't get consumed.
I don't think this is right-wing specific. You could probably draw similar conclusions coming from an Islamist angle. And this so-called AI is going to be next "frontier" is not all that clear to me yet. It's a nebulous threat at this stage that starts with a lot of "imagine if" arguments. We don't know yet. It's worth paying attention. But we don't know if HitLLMer chatbots are going to cause more damage than the concentration camp simulation games that preceded them.
There is a good 15% of people who are drank the koolaid right-wing believers. I don't think that number has changed much in the last century. The number that changes is how many of the less extreme or undecided people in the middle they can convince they're right.
The internet is only as regulated as the least regulating country on this planet. So all it takes is a tiny island nation or a principality left over in time to break the chain. It's also conceivable (imagine if!) that a fine, upstanding citizen like Elon Musk uses the change he found in his couch cushions to circumvent any regulatory efforts anywhere to distribute otherwise regulated content via his private satellite network. The answer cannot be "let's limit speech more." The answer must be "fight back with truth and facts." If asshole ideologists use all the digital tools available to spread their bullshit, we need to fund initiatives that counter that speech with the same tools.