Vue with Vite (the builder/runner) is a stable, open source option. It is really a lightweight start where you're mostly writing HTML with interpolated data, and Vue is updating values correctly and performantly. Just build your reactive HTML app in one file and break into separate components as you're feeling the spirit.
https://vuejs.org/guide/quick-startMithril if you just want to drop in want a tiny, complete reactive library that doesn't require a build step--this one is most like what you might end up creating in a large jQuery app. You can understand everything from the homepage. https://mithril.js.org/
HTMX if you really like HTML conventions. This doesn't feel jQuery-like and depends on your approach to your server app. https://htmx.org/
If you were to pick just one, I think Vue would be the most rewarding.
