I don't know much about gardening, and this is something that just came to mind, so take this as the half formed idea that it is.
I wonder how well using the soil from a raised planter would be at making the garden soil better. For example, if you used raised planters for a year or two, then mixed the soil from them into the soil in the garden. Maybe doing a section of the garden at a time and completing it over a number of years. Any compost you make would help too.
I know that obviously putting nutrients into the soil is going to be good for growing plants, but I don't know how helpful it would be if you could only do part of the garden at a time.