Unfortunately, 100mm filters are not wide enough to work with the 14mm lens. What I now need is 150mm filters, and I decided to invest in the NiSi 150mm filter system, which seems to be popular with landscape photographers.
Yesterday I ordered a filter holder specifically for my lens, a 150mm x 170mm 1.2 (four-stop) soft graduated neutral-density filter, a 150mm x 170mm 0.9 (three-stop) reverse graduated neutral-density filter, and a 150mm x 150mm 3.0 (ten-stop) neutral-density filter.
This lineup should nicely cover my needs.
Fortunately I was able to sell all of my other filters in one go (including some screw-on filters which were also surplus to my needs).