You have an issue with your arguments.
If Ukraine is truly sovereign, then their defense is their responsibility. This is evidenced by the fact that there are no legal requirements placed upon the US to assist Ukraine. Now, this does not mean that there can't be bilateral agreements with Ukraine to assist them, and there are - Biden signed it in July of 2024. But there is one small problem with it, per the US DoD:
Since there is no legal requirement to support Ukraine and they are sovereign, the US can stop support at any time as determined by the elected officials. This means support can be used as a negotiating tool between two individual countries that are independent of each other. IOW, there is zero obligation to continue supporting Ukraine since they have the right of self-determination.
If, on the other hand, they are NOT sovereign and are instead a vassal state of the US, the US has the choice to continue to support Ukraine or discontinue said support at any time in favor of other situations that favor the US. This would include allowing certain parts to be annexed by other countries by virtue of not providing enough support to prevent the annexation.
Both situations lead to the same conclusion: the US is under no legal or moral obligation to continue supporting Ukraine, especially if that support endangers the US, its citizens or its overall goals on the world stage. If Ukraine is sovereign then they should make their own decisions about the conflict instead of being pushed via NATO to cease negotiations (as it did in the initial phases of the conflict), and make their own way through this mess. If they are not sovereign then they are at the mercy of the US and NATO's decisions. However, it is noteworthy that Ukraine is in no position financially or otherwise to exert any power to upon Russia or NATO, the exception being calling up Russia and negotiating themselves outside of NATO desires, but they are not doing that. Instead they are feebly trying to do what NATO wants while NATO does very little and waits upon the US to supply leadership and resources, which is being reduced in favor of negotiations. IOW, we are now using our support as form of sanctions to push Ukraine to the negotiation table as Ukraine fully understands that without US support they will have zero chance at anything that looks like a decent outcome. Additionally, accepting aid from any country or the IMF comes with concessions regarding governmental directions and commerce - and those hooks set in deeply (i.e., Blackrock, rare earth minerals, etc) and last generations.
While it can be argued that Ukraine is a sovereign nation, the actions that are occurring are providing evidence that in practice it is very much a vassal state with ultimately little say in how events transpire from here on out. NATO is unable to assist them meaningfully and Ukraine is unable to exist effectively under the current conditions, even to the point of being unable to pay the government employees to ensure Ukraine can function as an actual country - it is closer to a failed state at this moment than it is to an actual country.
Ukraine has fallen in every aspect except legal status. Its over. There is no position of strength to be had, and while there is much talk, there is no appetite to fully support Ukraine by anyone in the world as they have their own economic headwinds to deal with (which are severe), and they don't want to open up a major conflict with Russia targeting them directly. So Ukraine will give up land, resources and autonomy in critical areas that define a country as being sovereign. This whole thing from 2014 to present day has been a folly of tragic proportions.