[pierrec]

It's hard to bundle without a bundler, but...

It sounds like preloading modules would help your case a lot: https://web.dev/articles/modulepreload

The number of connections is a bit of a red herring here, the problem is (typically) that the browser loads one module which then tells it to load another module, and so on. Each round-trip is wasted time. Preloading gives the browser a flat list of all required dependencies right away. By the way, this applies to everything else: having all required resources declared at the top of the page makes things faster. You can even preload some less-obvious things like background images referenced by CSS files.

This might achieve the "bundling" you want, in the sense that all the preloaded resources can be multiplexed into a single connection. But again, the number of connections is almost nothing compared to the number of round-trips required.