I’ve been poking at this trying to come up with some ideas, and I have a few follow up questions and thoughts.
Do you want a single HTML file that just loads in JS? You could do this with Parcel and get hot reloading when ReScript compiles or for changes to CSS and HTML. You could do lazy loading to break up the code into smaller files and import when needed. This is a very manual approach and navigation and routing would be tricky.
Do you want to just be able to deploy static files, or would you be OK with a server? HTMX might be a a good option.