I discovered that the Iosevka font has character variants to suit it to the way I want it. However, it uses (what I may have to call) “custom” OpenType tags such as cv65. So I tried adding to the user preferences file:
"font_options": ["cv01", "cv05", "cv11", "cv19", "cv21", "cv23", "cv26", "cv27", "cv36", "cv38", "cv45", "cv46", "cv49", "cv51", "cv52", "cv65"]
However, it did not work. It does, however, work with non-custom ones like ss01.