To build a SPA you need a couple technologies.
- A routing component, so your SPA app can navigate between pages. You will need to pick a jquery routing component. jquery is not as popular as it once was for SPA, so many of the components are old. You might do better googling for a vanilla js library.
- a template engine to be able to dynamically display your html. There are a lot of these.
- a build tool chain that supports js modules. You will need to build your 20 pages into one via bundling. Webpack is the classic, but rollup or parcel may be a good choice.
note: I use react and parcel, so can not recommend a jquery only solution. As a first step you might pick a build chain and convert all your JavaScript code to modules. This will make sharing easier.