I would check the Cityline Series (Vienna, Berlin for example) or Pia (also known as Pink Elf; big blooms; will need winter protection).

Sorry. I was going to list three answers and left out Masja. By the way, roseabbey, I assume your soil is well draining and that the soil Ph is not extreme. If it is, you will need to amend it once or twice annually. Being so far north, you will probably not need to pay as much attention as I do to the amount of sun that the plants get in mid-summer (to protect the leaves from frying). I hope others chime in with additional suggestions. Let us know what you choose. If you post a picture of the gazebo and close-by plants or structures, there are some people here that can also suggest other plants as well.

Here is a link that might be useful: Planting Hydrangeas

I think one of the forever and ever series would be the height you want and zone hardie for your area. My forever and ever Early Sensation has been a blooming machine for me. Its only supposed to get to 30 to 36 inchs tall and wide.