It is already good that you have him sit before going out. Sitting is a deferential behaviour helping him to gather information about what's going on. In addition sitting helps the dog relax.
Before going out for a walk you don't want to train for an "excited" or "obedience like" sit (I mean that the goal is not the "sit" in itself, like a trick). You want him to sit and look at you ("hey what's next?") while staying calm and relaxed. This last part prevents the brain from going in "crazy" mode (not very scientific...) which in turn prevents him from thinking and then learning (harness or collar fitting, loose leash walking, ...).
You seem to be doing quite well there, you can really enforce that in other situations (doors that you open, picking up a toy, etc.). Eventually you won't even have to ask him to sit, just wait until he does. If you do that consistently you also reduce his anxiety: he knows what will happen and he's in control of the situation (from his point of view) and understands that doing nothing is a good thing.
Collar fitting and 'manipulations'
This is to be trained separately: don't do that when you want to go for a walk. Do that when you're calm and relaxed.
The goal is to train the dog to have your hands close to his face, neck, etc. This is also valid for other 'manipulations' (you could practice with his paws, tail, eyes, ears, etc.). That can be very useful if you have to give him medications.
Take some treats, call the dog, ask him to sit. Approach your hands from his face, say "good boy" and give him a treat. Progress very slowly. The goal is to reinforce the "good" behaviour (not moving), so you need to create a situation in which that good behaviour happens.
When he's confortable with your hands then you can start doing the same with the collar. Progress slowly: that may means 3 sessions (1 session of 5 minutes a day?) without the collar with some dogs, or 10 sessions with other dogs. The progression is usually non linear.
You can give him a treat through the loop for the first few tries. However that's bribing not rewarding. Quickly fade out the treat and only reward after the good behaviour.
If he's moving too much or trying to snap your hands: don't say anything, stand up, turn away from him and/or move to another room. Wait 15 seconds and start again. In these situations you don't have to talk much: don't "command" anything (apart from the initial sit), don't yell at the dog, etc.
As he'll make progress you can also touch his neck in other circumstances, before feeding him, before petting him, etc. That will help him generalise and proof the behaviour (no reaction when your hands approach his face) with distractions.
Fitting the collar to go for a walk
That's the last step: the dog is calm and relaxed (part 1) and had some training about the collar, he's confortable with your hands being close to his face (part 2).
Don't talk and breathe calmly. You both know what to do. If he's getting nervous when you try to fit the collar, stand up, turn away from him and wait 10 seconds. He should sit down again by himself.
Long answer but that's not rocket science. The main point is the desensitisation and/using positive reinforcement of being manipulated near his head. The usual error is to try to progress too quickly. That's a problem I'm having now as I have to apply an ophthalmic ointment to my dog.
I'm talking about "saying good boy and treat", that can be adapted to clicker training (click and treat - see this video about clicks and calm markers) or any other positive reinforcement training method.