Direct incentives like this usually get gamed or don't work. I prefer the Georgist approach, working with, not against market forces. Make all land speculation fully unprofitable by taxation, and make all returns from labor and capital (i.e. wages, interest, and non-rent profit) free from taxation or rent. Use the difference to fund a citizen's dividend.
> Simply redirect all those agricultural subsidies from corporations to families.
Absolutely, all direct subsidies must go, but let's not replace them with other subsidies. A Georgist system would replace all rentier subsidies with a labor and capital friendly free market.
Cities/suburbs come with appealing perks: entertainment, dining, large dating pools, etc.
What percentage of young people could we really convince to give up those things and how much would it cost us?
I don't doubt that some young people badly want a rural/agricultural lifestyle. I'm skeptical that we could convert more of them with just money.
Pay kids to move to, or simply stay, in Kansas.
After the kids get some relevant degrees, give them zero interest loans, new tractors, whatever. I really don't care what it costs.
Simply redirect all those agricultural subsidies from corporations to families.