Caveat: I suck at JavaScript. "One-page-ish" makes me think Angular; which besides taking care of a lot of the underlying framework stuff, lets you write your code in Typescript (which eventually gets compiled into JavaScript). Based on the very little experience I have with it, I much prefer using Typescript. May not be of help for an existing app, but maybe worth looking into for the next one.
Which reminds me, I really should go back to that Angular course I got when Udemy was having a sale.