You can automatically make your radiation box by right-clicking your geometry and selecting "Create Open Region...". Put in the frequency you want and it will auto-create the air-region and boundary around your structure. It may or may not be a similar size to what you've already selected.
The size of your coil structure looks in the ~10-100 mm radius range. The size of your air box is many times greater than this, ~5000mm radius range, so 50x - 500x times bigger. Perhaps the meshing is not as tight as it should be near the coil geometry itself because of how big the air box is relative to it. You can try seeding your coil geometry with a specific lambda/N value so it has a smaller mesh. Maybe that will give you a more stable quality factor?
Seeding means you "Assign Mesh Operation" and select either "On Selection" --> "Length Based..." for surfaces, or "Inside Selection" --> "Length Based..." for volumes. The value you select is some fraction of the wavelength you're looking to simulate. The lower values you select for seeding the more RAM you'll need, more time the simulation will take.
HFSS is a lot better for higher frequencies than lower frequencies, but maybe there are better ways to use it for these lower frequency ranges.