Okay :) And apologies for there being some "speak to HV" in there but if you're working out as you type, which I def do as well - then maybe it's a useful thinking point anyway.
Speaking to a HV or GP does not mean that you are looking for a diagnosis or being neurotic, though. To me the reason to do this would be to say "I'm struggling with some difficult behaviour in my child, and I'm looking to rule out underlying causes such as recurrent ear infection, speech delay, hearing problems, and anything else I might not have thought of as I am not a medical professional".
They probably will ask how he is at nursery, which is why communication with nursery is a good idea. I would go to nursery first. (take notes/ask for a written report) Find out any patterns. What specific problems are they having. What doesn't work. What does work well. Any areas (social, impulse control, behaviour, what do they think?) where he's out of step with his peers. Take this info to HV and see what she suggests as next steps. I don't think this will have your card marked as an anxious mother, I think this will come across as diligent and organised. If they say no, everything is normal, he will grow out of it, then ask when would she expect you to see an improvement, and what should you do if there is no improvement? Explain that you are concerned about this behaviour persisting into school, and that you do not want your son to become a playground bully. That's your concern, isn't it? So you can just be honest about it. That won't come across as neurotic.
You also said you wanted a parenting course, they can refer you to one for free (and it will be a decent one) or I also linked to one much earlier in the thread, and someone posted the link to Incredible Years, which is another (similar) evidence based approach.
Forget diagnoses for a moment because I think this is causing you to get stuck in a loop of "but he doesn't have that and anyway if he does have that I can't do anything" (Someone else answered the "can't do anything" v well above). You said that you are wondering if this is temporary. It seems to me that he is a little bit stuck in a stage most children leave behind earlier, which might mean that he won't move out of this easily without help. I don't mean to paint a bleak picture but I do think that if you get stuck in an unhelpful cycle there is a potential (NOT guaranteed!!) outcome where he continues to act out, you and other adults - nursery, school, etc - and other kids, kick back, or dislike him and he ends up with this negative self-image and combative automatic defensive response. That's the worst case scenario and where you don't want to go because it takes even more work to unpick that. So how to change the story? Look at what all the challenging behaviour experts say. Not the normal parenting "experts" (many of whom are anything but) but the ones who go into schools and work directly with the worst behaved children who don't respond to anything else. (Ross Greene, Becky Bailey, Stuart Shanker, Alan Kazdin, Thomas Phelan, Mona Delahooke, various Occupational Therapists or behaviourists, for example).
And they basically all say variations of the same thing:
If you're repeatedly doing the same thing and it's not working, it's probably not the case that the child is refusing to meet your behaviour expectation, it's probably that they can't - there's a gap there between their ability and what you're asking them to do.
Most of them BTW say that if there's a diagnosis it's not necessarily relevant - a diagnosis will get you access to any therapies and medications (maybe) and an EHCP at school etc and it might give you info about what might be realistic in a child of that age with that condition, but it doesn't change anything about what you actually do. The process is the same, diagnosed condition, undiagnosed, no condition at all.
They all say that harsher/stricter punishment isn't the answer and generally makes the situation worse. I know you have not suggested this.
Then they tend to split off into what they do recommend, with a lot of similarities.
All of them emphasise building connection is important. Positive regard for the child is important. Catching them being good, and recognising this (with praise, rewards, attention or even just noticing) is important.
Many of them recommend getting really specific about the exact behaviour that you're expecting to see. This helps in a couple of ways - it means that you can see if it's really realistic. For example if you're currently at "unprovoked physical violence" and you want to get to "plays nicely/neutrally ignores other children" that's quite a large gap. You would not expect your younger child to potty train tomorrow, I expect. You know that there is a long way to go in terms of verbal communication, recognising their body sensations, bladder development, being able to physically reach the toilet and take on and off clothing. Difficult behaviour is basically social skills and this is the same. Breaking this down into more manageable steps, and working out what the required underlying skills are can be extremely helpful.
Some provide info about what is likely going on behind the behaviour. For example, both Mona Delahooke and Stuart Shanker, and a lot of OTs talk about the concept of stress-response behaviour. This one has been huge for me. I could see it happening with DS1 but had no language to explain it to anybody else. Now I understand what is happening.
Some differentiators for misbehaviour vs stress behaviour are that misbehaviour is able to be changed through normal discipline techniques, punishment, reasoning, etc. It's behaviour that is thought through and decided upon and has a sort of logic to it (even if it's childish logic or results in something antisocial) the child can reflect on what they were thinking at the time and why. You can interrupt it and the child might be defensive but ultimately will respond to discipline in an appropriate way.
Whereas stress behaviour tends to be extremely sudden and seem to come "out of nowhere". It can often be violent. If they are talking, their voice will be harsh and high pitched. They may even laugh. Their body will be tense, if you put a hand on them to try to move them, block them or just try to connect with them/calm them down you can feel this. Their face may have a strong expression, even seeming "evil" or "manic". Discipline attempts tend to be like adding fuel to the fire and ramp them up rather than down. It can often take a long time to calm them down, and by the time they have calmed down they have absolutely no idea what you are talking about when you try to discuss it with them. They either shut down and refuse to give an answer, give a silly answer, or literally don't seem to remember/understand. Discipline has no effect on it and it will continue to happen, often with a pattern of the general surroundings of the trigger, even if the trigger itself is somewhat of a mystery. It can be difficult to see the overarching pattern, which is why conferring with other settings/adults can help.
The explanation for why it is so sudden and out of the blue is that what you're seeing with stress behaviour is the culmination of dozens of individual, tiny, invisible stresses which are all feeding into a sort of "Buckaroo" effect on the child, where this one thing happens and it sets off a sort of explosion of all of the other things. You know like when you are having a bad day and your DH makes a stupid joke and you snap at him, whereas another day you would just roll your eyes and ignore it? It's basically like a very extreme version of that.