Increasing font weight in main nav, as well as removing underline from active link and inactive link hover states in main nav on 7.0 Flatiron template

Site URL: https://www.codylafond.com/

Hi all. Through a ton of trial and error I was able to make use of a custom font for the nav, however I cannot seem to increase the font weight beyond it's current state. The font in question is a variable weight Google font (Plus Jakarta Sans) which can go up to 800, but it seems to be maxing out at 400 (visual estimate) or so despite several attempts to specify the weight in CSS.

I'm also trying to eliminate the underline in the nav entirely from both the active link state and the inactive link hover states. No luck on that one so far either. 

My template is Flatiron on 7.0. 

Still working on the nav font weight, but I was able to figure out the active link and inactive link underlines using these code snippets:
 

active link

/* Remove underline in nav active link */
#nav .active-link a {
    border-bottom: none !important;
}

inactive link

/* Remove underline in nav inactive link hover */
#nav li a:hover {
    border-bottom: none !important;
}