As I see it, Ukraine's only option now is developing (or acquiring) their own nuclear weapons. A lot of the USSR's weapons development was done in Ukraine, so they're certainly capable of developing them, it's just a matter of how long it would take. Of course, either the UK or France could declare exigent circumstances and hand a couple of nukes to Ukraine - the way the US did with Israel (we all know about that?), so there *is* a precedent - and something done by the US to boot.
I'm pretty sure that the Russian invasion in 2014 would never have happened if Ukraine was nuclear - nevermind 2022. Time to fix that.