I agree with you that Russia is wary of a Ukrainian offensive into Crimea. The fact they are building defenses suggests that they think this is a possibility and are uncertain of their ability to control the area. However, in the picture you shared, the defenses are aligned along the beaches. This suggests a fear of a sea assault, not a land assault.

Taking a step back, I have always felt that the endgame for Ukraine was to cut off all resupply access to Crimea, both through the land bridges and the Kerch bridge. Once this is done, Crimea will be isolated and essentially will fall because of an inability to resupply its population and Miltary. I don’t think there’s a necessity for them to actually invade the peninsula.