You aren't failing your boys. Firstly you are acting to protect DS2 and doing the right thing being watchful and not leaving them alone together. As DC2 gets stronger he will give as good as he gets.
Secondly you recognise your firstborn is upset and is acting on impulse. There is an old saying, your child is not just giving you a hard time, they are having a hard time. This is an upsetting phase but absolutely normal, you won't be judged if your tell your health visitor.
I am sure you try and share attention equally, sometimes you can make DS1 feel reassured by telling your youngest, "You’ll have to wait a moment while I help X put his pyjamas on" or very obviously praising him to someone else while he's nearby so he hears a positive message.
From the moment each child's day starts, be sure to praise his good behaviour and remind him how much you love him with cuddles.
Is it possible to take both out every day for a change of scene? Burning off some energy will do them both good. picklemepopcorn asked about nursery. Is he attending one? Don't worry about him thinking he is being banished, he will have lots to do and children his own age to play with, the staff will monitor behaviour and will tell you how he copes. Meanwhile you will have extra time to spend with your youngest.
At home, next time you sense he is about to boil over give him a cautionary warning. Get down to his level physically when you talk to him, keep your voice calm and really listen to what he says.
If he persists or lashes out then comfort his brother, and immediately remove your eldest from the room or if you are away from home, stop whatever activity and take them to one side.
Fwiw my thinking on admonishing DS1 physically is this, he is acting on impulse against his little brother so even a little tap or spank from you during a telling off is signalling it's all right for bigger person to hit a smaller person.
It doesn't seem to have stopped him anyway. He is little more than a baby himself and still has trouble controlling his emotions.
Instead of removing several of the toys which does not always have much impact, pick one and put it in a "naughty box" for the rest of the day. He can win it back sooner by kind, good behaviour.
We had photos up of our eldest from birth and reminded him he too used to be a baby. I'd tell my eldest he used to do this or that when he was the same age but now he can do x or y so much better, he's a big boy, etc. It used to crack me up when he very seriously said to me, "Little kids don't know much do they", he himself so young but gradually grasping that his younger sibling would soon learn stuff too and not be so dependent on us.
I was advised to get him helping me when I saw to his baby sister; brimful of purpose and being busy, he enjoyed feeling useful and being praised.
If you want professional advice of course you must seek someone qualified, perhaps you could try some ideas on your thread in the meantime.