A tac nuke launch from Belarus into Ukraine would initiate some complex diplomatic and strategic issues...
Ukraine isn't NATO. Belarus isn't Warsaw Pact / USSR. It is, even if only nominally, an independent state.
Putin is keeping his nuclear option on his doorstep, but outside the homestead. Belarus is clearly expendable. Its a highly destabilising move, so he's playing his brinksman / unpredictability card again.
What If question: Ukraine was the home of the USSR military industrial manufacturing industry until its breakup. It also has multiple nuclear power stations and 15 reactors. It is currently in a war of existential survival against what is in reality a dictatorship in which opponents serve long prison sentences, get cancer, fall out of windows, eat pollonium sandwiches or get poisoned by Russian chemical weponry completely cooincidentally in the same week proven agents take a sabbatical to visit the Cathedral in their city.
Could they have cobbled together enough 235 to make a couple of simple 'gun type' nuclear weapons as a last resort, even if they are to be merely announced as 'a fleet in being'?
Israel, South Africa, Pakistan and India and North Korea all managed it....