He’s not spunky and opinionated, he’s a puppy.
Equate this to a child who is running around having a whale of a time, but you feel they are getting tired and need a rest. Are they going to calmly allow you to pick them up, or throw a tantrum?
You are basically stopping him from enjoying himself and he doesn’t like it. Please don’t turn it into a battle of wills, otherwise you are building up trouble for yourself.
As with children, you have to set the boundaries. Pick him up at other times so that he doesn’t associate being picked up with you stopping him doing something, make being picked up being something pleasurable. We always seem to believe that animals should have some in built mechanism that means they always comply with humans, no they are quite able to do whatever they want. The trick is to want to do what we want and as with children in a way that they think it’s their idea. They are much better at training us than us training them.
Rewarding the behaviour you want is the way to go. Reward what you want and ignore what you don’t, unless it’s dangerous of course. Cocker spaniels are very amenable, but stubborn and want to please you.
Enjoy your puppy, they soon grow up