Finally there – wanted to get it all tidied up, written and documented:
npm-free LiveReload(ish): Simple, depedency-free scripts for asset watching and auto-reload in the browser
https://rosswintle.uk/2024/03/npm-free-livereload-asset-watching-auto-reload-scripts/
This is how I'll build small, static sites locally now. Yay!
If you just want the code:
https://gist.github.com/rosswintle/d1f6428685be5eb1091dab2246015ff2