I think basically the only Russian plan that makes any sense is to crush a big chunk of the Ukrainian army in the east, then grab everything up to the Dnieper south of Dnipro, then hold the line stretching from Dnipro to Kharkiv (or somewhere just south of there if they can’t take it). The Russians don’t have the troops to hold a line all the way across the country with no terrain to help. Let alone adding a goofy land bridge that’s a bunch of additional frontage covering very little territory.