Struggling with our 3.5 year old

We feel like we're failing at this parenting thing. Our son is 3.5 and a hyper little boy. He can be such a sweet boy, but he can also be just so bad😩 my biggest concern is his aggression towards his 8 month old brother. He typically yells at him, rips away toys, or shoves him over. But yesterday for the first time he bit the baby, twice. And I'm absolutely horrified, the baby has perfect jawline bruises on his arm and calf. We responded with a time out, I stayed calm (but had to go downstairs and cry because im just so frustrated).

Everything I've read says they seek attention at this age, negative or positive, it doesn't matter. So we try to downplay the negative behavior with a quick no, this is bad because ______. Then a time out. And when he does something good we try hard to notice and encourage the good action.

We make a point to give him individual attention time. As well as we made a play pen area in the living room special for just him, the baby isnt allowed in, so he has a "safe" space for the toys he doesnt want to share.

I'm at witz end trying to figure out what I need to do to decrease this aggression. I'm genuinely worried about him harming the baby. I try not to leave them unsupervised, but sometimes all it takes is 10 seconds of inattention and he does something. And I want to make sure his needs are fully met as well. I just dont know what he needs 💔