As rigmarole suggested, we can easily add a p to .markdown-preview-view p { } to target paragraphs only, in Reading View anyway, but it shouldn’t be necessary here.
Using Anuppuccin and a slightly altered version of your CSS,
.markdown-source-view.mod-cm6 .cm-scroller {
font-family: 'iA Writer Duo S';
font-weight: 900;
}
.markdown-preview-view p {
font-family: 'iA Writer Duo S';
font-weight: 900;
}
I’m not seeing a font-weight change for the headings in Editing or Reading Views:
font-weight: 100 →
font-weight: 900 →
Also in your own CSS, I’d stick to numbers. These lightest, lighter, etc., seem to be defined in the Anu theme.
There are a bunch of Style Settings as well to play with, including ones for individual heading weights.
Just a guess, but I suspect your other heading CSS (defining the font) or some Style Settings may be conflicting in some way. You could try putting the custom heading CSS, with a font-weight property, under this CSS as that will be applied last, or poke around the Anuppuccin Style Settings.