The way you describe what you are doing it sounds like you know what you want at the end of a fairly long time frame, and your doing one set of pretty basic planning and then acting surprised when that plan doesn't give you what you want at the end of the project. This is pretty typical when groups try to transition to an agile development process, and fail as a result. All you've done is throw away your detailed planning and chunked your work up into sprints.
Try Scaled Agile.
It's what we currently use and is the best tool-box that I've yet come across. Things like SCRUM etc don't define process at the business level (intentionally), nor do they describe how you plan out a project that's going to take a year or so to deliver. Scaled Agile does, and it's a very useful toolbox for getting the whole show to be agile (which is the end goal for a business as it allows the entire business/activity to adapt quickly to changing conditions, both internal and external). At the very least, it's useful to read (all online), as it demonstrates all the extra bits that you would need to transition fully.