I think you hit the nail on the head when you said "as much noise as he's made."
While serving in Congress, what prosecutorial authority did he have?
If none because that rests with the executive branch, then what was his best course of action after uncovering criminal activity?
Making as much noise as he could to either force the executive branch's hand to prosecute itself, or make sure the American people (voters) were aware of the degree of criminal activity.
That appears to be what he did. I assume knowingly that he was possibly endangering himself and family members.
He has been noticably quiet for the past almost 2 years. Having a DOJ that he can make referral for investigation and they truly investigate I think is the reason behind this change. No need for public noise, if lady justice with blind scales has returned.
Congress has the power to investigate. And it has the power of enforcing testimony and contempt- see below. But this power does not expand past this limitation due to Constitutional separation of powers.
https://fas.org/sgp/crs/misc/RL34097.pdf