A 'Treaty of Versailles' type solution is not a good idea for durable peace though, harsh reparations, despite any sense they might be 'fair', seldom lead to both countries returning to be prosperous democratic countries (and to be clear, neither is a capitulation by Ukraine - that would be seen by Putin as locking in its current gains, with no real incentive not to try again for more despite what the treaty might say).

The best outcome for everyone is if Russia ends up being a genuinely pluralistic democracy (i.e. anyone in Russia can have political views, and the public selects its leadership in free and fair elections). Then Ukraine can normalise relations with Russia, and Russia stops being a threat to democratic institutions across the world as a whole.

I think the best way of thinking about it is not that Ukraine has a Russia problem, but rather that Ukraine and Russia have an oligarch problem (with Putin chief amongst them). Therefore, in a fair world, the oligarchs, and not the Russian people, would pay. It is true that Russians (and indeed some Ukrainians in occupied regions) have been radicalised by the oligarchs, so some kind of deradicalisation would be needed even if the oligarchs disappeared.

Solutions that look to negotiate how to reduce corruption and authoritarianism in Russia from the top are therefore the most likely to succeed long term. Shorter term solutions could include a negotiated end to hostilities coupled with agreements for Ukraine to join a defensive alliance that the oligarchs wouldn't consider provoking - which could be followed up by a carrot approach to easing sanctions in exchange for progressive movements towards genuine Russian democracy. This might give oligarchs enough push to take off ramps to cash in what they have plundered already, and slowly be replaced by less corrupt alternatives going forward.

Recovery from oligarchy for Russia might also by costly for Russia though - essential assets plundered from the USSR are now in private hands through crony capitalism; the best solution would be for many of the major ones to go back to or be rebuilt under state ownership, under genuine democratic leadership. But that is likely easier said than done given the state of Russia.

And a 3rd party enforced DMZ on the border.

If only you also were in the position to dictate this to Russia. Even the US isn't in this position, and will never be.