First, you're never going to see monetary rewards for minor league achievements. That would just encourage owners who know they're not going to competitive at the ML level in a particular season to demote marginal ML players with options, or sign unsigned ML free agents and assign them to the lowest levels of the minors to try to achieve $$ credits while they are rebuilding at the ML level. That would be a very bad thing. And it WOULD very definitely happen.
Second, you can be successful and competitive in your minor leagues while letting the AI manage your teams, with only minimal manual oversight. It's just a matter of (a) having enough extra (i.e. inactive) players on your roster, (b) having the right managerial settings, and (c) not getting too "Type A" personality about individual player results. If you would like some tips, please feel free to contact me via sitemail.