The description excerpt truncation happens on the server here.
It’s limited to 300 characters, and there’s no setting for that. However, you can use a theme component to bypass that and override the property and make it use the full description (the entire first paragraph).
Try adding something like this in the header section of a theme component.
<script type="text/discourse-plugin" version="0.8">
const { on } = require("ember-addons/ember-computed-decorators");
api.modifyClass("component:parent-category-row", {
@on("init")
updateDescription() {
this.set(
"attrs.category.description_excerpt",
this.attrs.category.description
);
}
});
</script>