We already have a real-world example of that[1], which is in medieval times in China, the land was a lot more fertile than in Europe, and with a crop also more nutrition dense (rice). So what happened there is that, instead of achieving something similar to the Shire, the plots of land became smaller per family, to achieve again an equilibrium where each family had enough to eat for themselves, but not a lot to spare. So more fertile/nutritional crops, while it would def help the Shire, would not be the whole answer without some sort of population management.

[1] https://acoup.blog/2020/09/04/collections-bread-how-did-they...