I still do not see why Russia would need to invade further.
It's not really about "need" per se, Putin and chums still have an old mentality re: spheres of influence and Ukraine is in Russia's sphere of influence as far as he's concerned, Ukraine getting closer to the West isn't something he's a fan of, the guy is still bitter about 1989, the fall of the Berlin Wall and the subsequent downfall of the USSR.
In practical terms, Crimea has a water issue thanks to Ukraine shutting off the canal so I suspect IF there was an invasion then getting a land bridge to Crimea, solving that water issue and perhaps expanding the two Eastern rebel-held areas would be conservative goals.
As we've seen with Georgia invading and causing damage then later pulling back is something Russia is happy to do, he invaded and took several Georgian cities before later withdrawing in 2008. In this case, there was recently allegedly a plan to install a puppet regime in Kyiv, perhaps there still is.
He certainly has enough troops to keep open the option of invading most of the country including taking Kyiv if he wants to, obviously holding onto the country post-invasion is another matter entirely but I presume that (aside from perhaps forcing regime change) he probably doesn't intend to.