The website is a replacement for an old website (written in 1997). For some reason, Google is still trying to index the old and now non-existent links from the old site regularly, while it only attempted to index the new, working links once 2 months ago. Needless to say, all links on the domain besides the landing page are considered duplicates, since they only contain the bare index.html page according to Google. The dead links still show up when I try to google the domain.
It is worth noting that I have both a valid robot.txt file and a sitemap displaying the valid links on my website. Search Console is not reporting any errors and it appears to have been reading my script files properly. Using the Google-bot simulator, I'm also only able to see the bare index.html page. For what it's worth, Bing and Yahoo give the same results.
According to all the sources I've found, Google in 2019 should do just fine with both SPAs. Since I have no errors to go by, I don't even know how to begin diagnosing the problem.
Even if you can't solve my problem, even pointing me in the right direction would be highly appreciated at this point. I don't feel like I have much to go by.