Assumption on your part. This only works if the cost of keeping what they’ve taken exceeds the benefits. So far that’s not the case.

It's not a "moment in time" calculation. Russia will make an assessment not just of what the costs are today, but the future costs throughout time. As is frequently the case, the invading country presumes that it will be able to eventually defeat the opposing forces conclusively. If they fail to defeat the opposing forces for a long enough period of time, that presumption is no longer valid - and they have to reassess the costs and benefits. Russia is still in the "we can still beat Ukraine" phase of assessment. That will eventually change, if they are unable to beat Ukraine for while longer.

If Hungary is the problem, address Hungary.

How?

You’re essentially laying out the case why the EU is completely useless, btw.

It's not useless. It's just not made for this particular purpose. The EU isn't itself a country made up of constituent states (like the U.S.). It's a collection of countries. Those countries have delegated a fair amount of power over their financial and economic systems to the EU (for example, erasing import/export boundaries within the block) - but they have not delegated nearly as much authority over foreign policy. It's an economic union, not a foreign policy or military union.