I’m sure I’m oversimplifying, but I notice in your about.json snippet, the path to the fonts points to an assets folder, but in your css, the path points to a fonts folder. Are you sure the css source path is correct?
I played with this a bit on a fairly new Discourse instance, and had some trouble with the first few fonts I tried. I had converted them to .woff2 using a free, online converter, and I think maybe it didn’t do a good job.
When I tried with an .otf font and again with another .woff2 font converted by a more reputable converter, I was able to add the custom font using the steps in guide @pfaffman and @awesomerobot linked.
It really only took two steps - uploading the file using the Add Upload modal, and adding just the @font-face and body lines of the example SCSS to the theme or component.
I’m sure if you can tell give us more detail and share your code and the fonts, we can help you get this figured out.