Your narrative, and a few other posters, of how you think our dog became aggressive is misguided.
I'm not here to engage in arguments, I asked for advice and I am prepared for judgement etc. This is my fault for not explaining, but I only put the main points in my OP, some more detail if you have time to read:
The dog came to us at 8 weeks. He was socialised as much as possible; minimum of 2 long walks per day, gatherings in the garden with family, our circle is small.
Breed awareness. I have awareness and experience of the breed. And with this awareness, I also knew the importance of the key socialisation window. I also knew that my dog needed a high level of exposure to people and dogs as he was a nervous pup, which we couldn't give him. Also know how they can be very frightened and territorial dogs and the need to socialise to prevent this escalation. From 12 weeks old, he would bark at strangers who we could not build him up to at that time. I'm talking the same time we had to queue a metre apart to get to Tesco time.
Training: He attended his good citizen bronze dog training award at around 10 months old, again, this was in a group setting and he at this stage would bark and lunge at strangers and dogs in the class. The class could only run outside due to restrictions and again, he was kept a metre apart.
Trauma: when he was 1, he had to have an operation on his back legs for luxating patella. This reinforced his fear of humans as it was traumatic for him.
Chihuahua Rescue: I've phoned multiple and all have said he wouldn't be rehomed due to his nature. I've gone online and advertised him with full disclosure and have only engaged with this potential adopter as she has provided her details and shown me her website with past work as a dog trainer. I am also aware this could be too good to be true, she could be looking for dog bait etc.
Also aware he might not get along with another dog. Her theory is, based on her experience of rescues, in a controlled environment, he may settle with slow introductions to his new pack and this might help him feel more safe and secure. She is visiting our home and if I'm happy and she is wanting to take him we will plan a home visit to her house. It's never advisable to go online and I know this, however, I'm exhausting all options.
If you've read this far, Apologies for the essay.