let's look at the historical case.
Finland -
. Now wants to join NATO.
Sweden - declared neutrality in 1949. De facto member of NATO that participates in NATO exercises, contributes to NATO missions and even allows NATO troops to be deployed to Swedish territory.
So it turns out that at most a treaty is 50-60 years and only if you're a superpower. Otherwise a treaty's not worth much.
Nope it's not worth.
I believe these are the viable choices for Putin in decreasing order of ambition
1. Installing a Russian dominated puppet in Kiev to rule whole Ukraine except all of Dontesk, Lugansk, Crimea, Kherson and Zaporizhzhia oblasts which are ceded to Russia (Ukraine is cut off from Sea of Azov)
2. Forcing an unequal treaty down Zelensky's throat, partitioning Ukraine and taking everything east of Dnieper and maybe Odessa and Mykolaiv and forcing neutrality on them.
3. Unilaterally partitioning Ukraine in the same way, ending in an armistice like the Korean War.
Russia cannot even accept merely taking Dontesk, Lugansk, Crimea, Kherson and Zaporizhzhia oblasts without a puppet in Kiev because there isn't a natural barrier to protect Kherson and Zaporizhzhia oblasts. It'll just be moving the Dontesk/Lugansk line a few hundred km west. That's not going to cut it because Dontesk and Lugansk have already been shelled continuously for 8 years.
It has to be a maximalist, all in, everything east of Dnieper if a puppet can't be installed.