When the war is over, it may be that the new-found unity caused by the external threat will be enough for them to reinforce their institutions significantly. This could reach a point that could ensure lasting democracy while enabling them to properly fight/eliminate most corruption. At that point, combined with a peace treaty with Russia, they should be able to Join.
More likely, Ukraine after the war will be a massively destroyed country, with widespread poverty and a constant need for western powers to pour money into - and that breeds corruption. The only way forward - should they survive as a nationstate - would be some kind of grotesquely enlarged Marshall Plan - they would become in the 21th century that what Germany was between 1946 and 1990: A military bridgehead and designated battlefield should Russia get another taste for war, with modest economic growth.
Still, most countries had rebuilt their economy to pre-war levels before 1955, and in many cases around 1950.
Given the very small size of Ukraine's economy, reaching their pre-war level will not take very much. Should they, as I predict, come out of the war as a united nation, able to get rid of most of the corruption, they may be able to massively outperform their pre-war gdp fairly quickly.
If they return to pre-war levels of corruption, no amount of economic aid will bring their economy to western levels.
https://www.bloomberg.com/news/articles/2022-05-05/u-s-gives...
Imagine diverting 50% of Russian troops from Ukraine, where they are already struggling, to another special operation in Finland. Can't see it happening.
They'd have to actually declare war to conscript the whole population and what nonsense could Putin say to sell that to his people at this point?