@roo2018
I think you're better off putting the dog to sleep. Assuming a vet check has ruled out chronic pain or other potential triggers, your dog is showing classic fear/phobia behaviours which are generalising. A good trainer should recognise that and know that fear cannot be trained out. You obviously don't understand how behavioural meds or clinical veterinary behaviourists work and it's easy to dismiss something you don't understand but if you are interested in developing as an owner /trainer I'd suggest keeping an open mind and being open to learning. One of the reasons you're having problems is because you're wedded to a faulty premise:
The idea is that the dog looks to you instead of reacting on instinct.
Yes this is good general impulse control training. It doesn't work for fear responses though because the amygdala is activated during fear and will ALWAYS override frontal lobe processing which governs intentional behaviour. A dog with an activated amygdala can ONLY react - they cannot cognitively process or learn. in order to facilitate cognitive control you have to reduce the fear response - this can usually only be done with medication.
I’m not new to this. I’ve researched until I’m blue in the face and haven’t found a shred of evidence that behaviourists are better at rehabilitating aggressive dogs than trainers with years of hands on experience
Then you'll likely be familiar with everything I'm saying already, but If you're referring to fear aggression then there's a lot of peer reviewed literature - so much so in fact that clinical trials have been done and based on this evidence, licensed veterinary medications have been produced.
If you search google scholar for dog fear aggression medication you'll find many studies demonstrating that dogs who are properly diagnosed and their fear treated appropriately have faster and better treatment outcomes. This is where clinical veterinary behaviourists have an advantage. Trainers are not trained in neurobiology, neurochemistry, emotional or motivational states, pharmacology, endocrinology or any of the other scientific subjects required to effectively manage behavioural pathology. Out of interest when was your dog neutered (is he neutered?) I'm sure you'll be aware of the impact of the sex hormones on emotional maturity and fear aggression - this is a common problem in dogs neutered before they're emotionally mature. Understanding the interplay between all of these different systems is essential to successfully managing behavioural pathology. It really is the difference between a human patient seeing an occupational therapist versus seeing a psychiatrist for chronic anxiety.
Of course you may want to dismiss behavioural science as 'textbook stuff' but both the scientific evidence and the practical evidence is clear. If you choose to ignore it, that's your call.
In any situation, there's always the potential for a therapy to fail and owner compliance is the biggest requirement for the success of a clinical behaviour modification plan. if you don't fully commit to a clinical behaviour plan then the risks of failure are magnified. In this case, it does sound as if it would be an uphill battle for many reasons - it sounds as if you're determined to value your own opinion and experience over years of scientific and medical evidence and expert consensus. You're obviously tired and a bit scared of your dog Which is of course not a situation anyone wants to be in. and quite right in that dogs with complex disorders including anxiety disorders often need longterm management. Most chronic psychological disorders aren't cured but managed and so realistic goals are necessary.
You don't say what breed your dog is or where he was sourced from but assuming a breeder I'd suggest feeding back to them about these problems if, as you suggest, there may be a genetic component further breeding from his parents would be irresponsible.