You need to wait until everything leafs out and settles in and assumes some semblance of maturity before you decide to move things around or add anything else. One of the most common mistakes homeowners engage in when planning/planting a new landscape is overplanting!! It takes time for things to fill in properly so you need to be prepared to be patient for at least a full growing season.....things will be sparse looking during that period........but that's perfectly fine!! Add some annuals if you think it looks too empty for now.

I agree with gg. Hydrangeas get quite large. Take a measuring tape and mark off a circle from the center of your hydrangea plants to see the amount of ground they will cover when mature. Is there any space left for the hellebores in the unmarked space? If not, use annuals in that empty space or easily transplanted perennials that you know will need to be removed as the hydrangeas get larger. Other placeholders can be large winter decorated pots of evergreen boughs, plants that will survive in your winter, dried seed heads; or sculptures, or interesting wood branches etc.

If you decide to use astilbe as a placeholder plant I suggest you plant them behind the hellebores due to heights. If the hellebore foliage stays nice during the summer they will make a good border in front of the astilbe. Keep in mind that unless the astilbe foliage does not disappear in the winter like it does in my zone it will not help with the bare look in the winter.