This is all about training your dog as well as using reverse psychology:
I have been around dogs since I was a child raised on a farm where dogs are a working partner in our world. I have rescued many red zone dogs ready for euthanasia and haven't failed Yet!
The main problem is that nowadays, dogs are bored, and lack training. I see you're eager to sort it when you mentioned treating your dog when behaving properly.
You're on the right path.
First, you need to have a "proper" relationship with your dog. Like a child, a dog needs boundaries. Your dog needs to understand what the rules are in order to respect how you want him/her to behave.
It doesn't mean violence as most people think. You need to look at what your dog does daily and ask yourself if you agree with it or not or should you need to put rules in place.
The example I alway use is getting on the sofa. In my opinion, dogs can get on the sofa as long as I allow them to get on. My dog will never get on unless I invited her on it.
You need to look at your dogs behaviour and ask yourself what does he/she do without your permission?
Training:
You disagree with a behaviour, say"no!" And mean it! It's all in your intent. The dog will challenge you. You must stay strong. It's a battle of will.
You disagree with a behaviour but it's trivial: ignore the dog.
Whatever you do, it needs to
Be followed with a replacement behaviour: "no!" This is what you do instead. If you get it, you get a reward for good behaviour.
Passive training is also great: if your dog does something you agree with, naturally, reward straight away. Dogs are watching us and are the only animals who can mimic, learn, and understand human behaviour.
You do that, you'll start to have a really good relationship with your dog.
This will solve your dog's aggression, taking the initiative. And by the way, your dog is probably leading because you're not.
The more training, the more interesting your relationship with your dog will be.
For the bell, hire help from a friend. Get them to ring the bell, let your dog do what it does and say"no" and mean it. Create an area, around the door that is yours and claim it as yours. It takes practise. Make sure the dog understand it's your space. If the dog wants to come in the space you're claiming it needs to wait for you're OK.
As per reverse psychology, if you want to stop your dog from barking, teach it to bark on command.
There are lots of books on training.
