I haven't really studied Soviet history post Nikita so idk anything about Leonid
Nothing. That is, doing nothing is what made him a revisionist. He didn't undo Khrushchev's policies either on the economic side (at least not sufficiently) or on the political side (no rehabilitation of Stalin).
He was just kind of bad at management. He didn't see the importance of computers and didn't have good policies to reverse the stagnation.
Didn't USSR have like 5% GDP growth per annum during the Brezhnev stagnation? (not that GDP is really an important economic indicator)
I read that alot of Brezhnev's policies were a return of capitalist elements, such as allowing economic enterprises to "fail" if they were unprofitable and making housing more market-like.
In a vacuum, I don't like these policies, but maybe they make more sense in context, I'm not sure. I know some limited reforms in the housing market were helpful.