Will offer my way of fixing.. probably something went wrong in the themes, aside from css, probably version mismatches with the core nodebb version your using. The below will revert to working vanilla and lavender theme versions that match w/ your core nodebb install version. (probably need to stick with whatever theme versions it reinstalls, because I've always had problems w/ updating themes, I don't upgrade them anymore) Stick with what came with the nodebb install originally. This will revert back to originals:
./nodebb stop (close nodebb)
npm uninstall nodebb-theme-lavender
npm uninstall nodebb-theme-vanilla
npm install nodebb-theme-vanilla
npm install nodebb-theme-lavender
./nodebb start. try avoid editing too much .less and .tpl in the theme files and stick as much as possible in the Custom css area in your ACP
I was looking into trying to figure out server costs with an estimated number of people, but figured it would be best to get some real world data to compare it to! So if you don't mind I would love to hear about your server specs/setup and how many users that handles!