Hello and welcome to COL
Sorry to read about your problems with your Cocker, and thanks for providing so much information about what triggers his aggression. None of us on here are behaviourists, and even with lots of detail being given it's impossible to give definitive answers on a forum. But there are plenty of us who have personal experience of dogs with problem behaviours and it may help to share those and the solutions we've found.
To me it sounds like your dog is fearful in certain situations. One of my current two Cockers is unable to cope in some situations since an incident when he was a puppy. A lot of his fears are around other dogs more than people, but his reactions are similar to those you've described. He will bark, lunge and snarl if he finds himself in a situation he can't handle and where his fear levels have rocketed sky high. He doesn't set out to be aggressive, but can't help his reactions when he's got to the point of being unable to remain calm.
We're a quiet 2 adult household and have lots of empty New Forest spaces to walk in. So we've opted to manage situations for him by not putting him in them in the first place where possible, but also by teaching him to stay calm. I understand that in a household with children and visitors, and perhaps busier places for walking, that it probably isn't as simple for you.
Our reactive Cocker, Louis, is almost 7 years old and is still entire. We've considered having him neutered but it's recognised that fearful dogs may become even more fearful once neutered. So on balance we've decided to leave him entire. He's not interested in the ladies and doesn't scent mark indoors or show any other undesirable behaviours that neutering may stop. Is it the right decision? I suppose we'll never know!
At home he's a bit nervous and barky around visitors, so we shut both dogs behind a babygate until visitors have settled in and then (depending on the visitors) let the dogs meet them. If Louis chooses to stay away from them and relax elsewhere that's fine. I'd never push him into meeting people or put him in a situation where he couldn't get out of a room with visitors in it because that would make him fearful. It's finding that mix of giving him the choice to say hello if he wants but at the same time never putting him in a situation that might trigger his fears.
On walks, we manage things carefully for him and try to avoid large groups of people, noisy children running about, groups of dogs, groups of cyclists etc. We know the distance at which he feels safe and we praise and reward when he shows calm behaviours such as looking and not reacting. It teaches him that he can stay calm and that nice things happen when he does. He's gradually become able to cope better because he's learned that he can stay calm even when the things that trigger his fears come quite close. So we have to manage his walks less than we used to.
We have setbacks and bad days sometimes, but if he reacts I've learned that I have to stay calm for his sake. So I apologise to whoever he's growled and lunged towards , explaining briefly that he's fearful, and walk him away calmly. He's worse on lead but of course I have to put him on lead sometimes if there's a situation we can't avoid. I'm very conscious that changes to legislation a couple of years ago mean that our dogs can be in trouble if their behaviour makes anyone feel threatened. So even a bark and a lunge while on lead would be investigated if reported, let alone chasing or worse.
I suggest that you seek advice from a professionally qualified behaviourist who will come to your home to meet your dog and all of you, and observe him at home and outdoors in order to assess the problem and help you find a way forward. A good place to start is the Association of Pet Dog Trainers (APDT) because they only use positive training methods. If you're on Facebook there's a group called Dog Training Advice & Support that you can join. They should be able to help you find a behaviourist in your local area.
