[Revolutionist]

What exactly did Lelouch accomplish?

absolutely nothing.

Here's what happens post ZR:

unemployment soars throughout Britannia as people whose livelihood depended on the military industrial complex see their jobs disappear. No more constant conflict = no need for massive numbers of sutherlands, tanks, ships, soldiers. People start to riot, and then it is only a matter of time until someone with Machiavellian ideals uses that to his advantage and makes a play for power.
They could use true Revolution as a pretext, or simply be a smart noble who bid his time and hid his wealth from Lelouch.