Ukraine has lost nearly a dozen Western tanks and presumably thousands of men already just to advance a few kilometers through the Russian screening zone. They have not even reached the first trenches

Not zero, that would be absurd, but much less than Ukraine. There is a very simple rule of thumb for estimating casualty ratios in such conflicts, and that is they are directly proportional to the artillery overmatch since most casualties in conventional modern conflicts are inflicted by artillery fire. Ukraine has been stockpiling their best equipment and munitions which they received from the West for this offensive, and as a result they are managing to shoot a fair number of shells and missiles in order to try and soften up the Russian lines, but Russia still has an overwhelming advantage. In addition Russia also has almost uncontested air dominance at the moment which further skews the ratio in their favor.