Large flowerbed - starting again

I am fairly clueless about gardening. We have one large flowerbed about 5x4 metres. As nothing grew at all well this year (and had got worse year on year) DH has dug everything up and it’s now a blank canvas. The soil is desperately depleted which is why nothing grew well for last few years apart from alliums and snowdrops and has been quite neglected as it’s not very accessible. He did discover a skeleton of what I think may have been a cat! He will hire a rotavator.

Does any one know what should be dug in to improve the soil quality and nutrients. Any ideas for plants? A rose and a recently gifted camellia will go in and hopefully a rather sad looking magnolia which is unhappy in its pot. Ground covering plants would be ideal. It’s quite shady as there is a line of shrubs (more like trees) at the back. We face south west.

Ideas gratefully received.