SEO doesn't have to mean repetitive keywords--you'd benefit from really simple changes that are also just good, accessible html markup. Make sure your images have titles and alts, throw up a solid metadescription, tweak your robots, submit xml sitemaps, maybe put up a breadcrumb system to reinforce internal linking and increase UX. Just good concern for the UX and proper markup will have a large SEO benefit anyway, no need to go crazy worrying about keyword density on-page. Your content is so good that you shouldn't have any problem getting backlinks (off-site SEO) either, which is where I'd spend my time after cleaning up the markup and making sure the site is really crawl-able and submitted to every engine properly.