Htmx is great for developers who need client side interactivity, but would rather not write any js. If you don't mind writing a bit of js however, you can't go wrong with Preact. Same api as React, but at a fraction of react-dom's bundle size. It looks like the minified source is even smaller than htmx (which is already very minimal) [1][2]. They've also packaged Preact in such a way that you don't need to add a build step to use it [3].
[1] https://unpkg.com/browse/[email protected]/dist/
[2] https://unpkg.com/browse/[email protected]/dist/
[3] https://preactjs.com/guide/v10/getting-started#no-build-tool...