Russia's agricultural potential is not currently limited by land, but rather by weather, which meets the same condition as you describe for Ukraine. The biggest advantage of stealing Ukrainian farms is an already established industry, which Russia needs to destroy in order to take the land. There are some arguments for destroying Ukraine to eliminate competition, but they are not that strong when you consider the global market demand, and if you include agricultural inputs such as fertiliser, which put the Russians into a very strong position. Russia also already has several valuable access points for the Black Sea. It would have been cheaper to develop existing ports for more deep water access than to destroy the RU economy in a war. Some more rail development would be necessary as well, but that isn't hard for Russian industry. As we have seen lately, occupying Crimean ports and airfields isn't as big an advantage as it used to be because warfare has changed.

The biggest reasons for taking Ukraine are cultural egotism. To redefine the Russian nation as a strong player in the world, instead of the fading gas station. There is something to say about one person's desire to create the historical narrative as a despotic conquerer.

BTW, Vlad Vexlers videos are quite informative from the philosophical perspective, getting at both the philosophy of the Russian leadership and the historical cultural perspective of the Russian nation.