I am a revert. Never said the shahada in front of any physical witnesses. First time I said it was when I prayed. Should I say it with witnesses or is it enough only Allah knows?

It is sufficient to say it in private. You don’t have to say it publicly. It would be nice if you gather some friends and say it publicly in front of them so you can form a stronger brotherhood with them, but you are not obligated to so do that.