I would use mulch, preferably leaves, but not soil. And do not do raised beds; the trees will not thank you for smothering their roots at all. The area under the maples is likely to be the most difficult since the thick mass of fibrous maple roots suck all nutrients and water from other plants, and if you attempt raised beds, the roots will just grow into them. In addition, even a few inches of soil can kill some trees. Just start planting. You may need to add organic soil amendments, but add nothing until you have your soil tested. Mine is sandy and nutrient poor and needed rock phosphate and huge amounts of leaves as organic material to hold moisture and build soil, but over the years it has become a gorgeous shade garden.

In my experience, bergenia wants much more sun and is probably not suited for your shade garden. Definitely look at Hellebores and Epimediums--many, many cultivars of the latter, both clumping and slowly spreading types. If deer are not a problem, Solomon's seal are grand elements in shade gardens, and there are a number of species. Also wonderful are Dodecatheons, Erythroniums, Cimicifugas (now Acteas, I think), just to mention some of the more common plants. And of course, ferns.

Good luck.

Thank you, laceyvail. The maples are at a distance on the neighbours' properties or at the other side of the house, as are most of the trees so I am not too worried about damaging roots. I had hoped to shred mounds of leaves in the fall but they did not fall off the trees until I had to stop the gardening for winter. Lots to deal with in the spring! I am quite excited to get going. I hadn't thought of Dodecatheons - good idea. So pretty. There are so many opportunities in a shade garden.