Would echo saying he sounds like my eldest who has ADHD and my middle one who is on a waiting list for some kind of diagnosis (ASD/ADHD probably) now at 5.5. If you have any suspicions here it's worth asking your health visitor, GP etc as the waiting lists can be horrendous. It might be he doesn't meet any threshold for referral just now but it can't hurt to ask.
In the meantime:
Big Little Feelings and anything from instagram etc - just forget it - none of these go into the right amount of detail and they oversimplify to the point nothing is helpful. Actually, TheOTButterfly on instagram is occasionally brilliant/spot on. But in general, instagram is totally useless for parenting advice. Avoid at all costs. Just makes you feel shit. Stick to podcasts and books - the longer format is more useful IME.
When Your Kids Push Your Buttons is on kindle for 99p at the moment - this is a really good helpful book, it helped me separate some of my emotions from my eldest's behaviour.
I also like How To Talk BUT with a caveat:
Mona Delahooke or Dan Siegel or Zones of Regulation or Conscious Discipline (Becky A Bailey) - any of these which describe the different emotional states are the missing key piece of info. Most the How To Talk stuff, and a lot of general parenting tips, tricks or advice are designed for when children are struggling slightly - the level of "stress brain" you tend to encounter when something underlying is really bothering them is another level, and none of these tips will work. If you think of the difference between a child being a general child and being a bit stubborn and self-centred and over-emotional in a totally normal age appropriate way, vs a child who really needs a nap or is "hangry" then you know, absolutely no sense or trick or anything will get through to them until they get that sleep/food/etc that they need.
Some children are just operating at this level a lot of the time, with no obvious cause, and it is exhausting, both for them and for you.
So it helps IME to recognise when they are just being a kid and so the standard tips and tricks might be useful, and when they are actually overwhelmed and in a stress state, in which case they will be as good as a chocolate teapot. (overwhelmed might not mean crying and tantrumming - it can mean stuff like manic laughing, "hyper" or silly behaviour, being deliberately destructive, arguing/defiance over everything, rudeness towards you, making unreasonable demands, trying to control others, winding people up on purpose - and you just sort of have that spidey sense of dread that challenging them is definitely going to lead to an explosion).
The only real thing to do in that moment is to keep everyone safe (which might mean you sit down and hold him on your lap facing away from you), de-escalate, which might mean moving all three of you to the side or a quiet corner in order to contain any explosion, and focus on body language not words - if you're not needing to restrain him, get down to his level, take some deep breaths (ideally he will instinctively copy you, don't ask him to), speak slowly and softly, don't demand, explain, reason, admonish or plead. Try to reflect what you think is going on for him so that he sees that you see/understand him. e.g. the "Validate feelings" stuff from How To Talk can be good for this. Also Janet Lansbury has some great things about "I won't let you [hurt your brother]". There's not a lot of teaching going on in this moment when they are so overwhelmed which caused the behaviour in the first place, just reiterating the boundary and trying to cool down. Some children are then receptive to a kind of teaching/learning conversation and/or compromise/problem solving conversation after they have calmed down, whereas some are just exhausted at this point and won't be open to it. In which case, don't worry - there are proactive ways to teach rules, skills and behavioural expectations too. It's not always about what you do in the moment.
There is a good course on Coursera (free) called The ABCs of Everyday Parenting. This is useful because it's a very positive approach, which is much more successful with children with challenging behaviour compared to an approach based on punishing what you don't like, but it's much more targeted and specific, so although it includes very basic-sounding aspects you're probably already using like praise and reward charts, I think it's helpful in getting those techniques to actually be effective where they maybe currently are not.
I can add more but this is really long so I won't - hopefully some of it is helpful.