and it's something that I think they would have done had the terrain been more cooperating as driving "off road" has been hard due to mud.

With a mech force, ground infantry aren't going to sneak up as machines are faster than human legs. The infantry will set their ambushes but out in the open, there are less places to hide (buildings) for effective ambushes while in the open terrain, there is less likely chance they will be ambushed because on the roads, the Ukrainians could pick their ground. In open terrain, the terrain and size of the battlefield lessens the likelihood of an ambush site that is the most favorable to the defense.

Getting into the open terrain will also help the Russians with encirclements and hitting the flanks.