Even just "make it a PWA" can take an experienced frontend developer a day or more. The manifest needs to be just right, the server might need to be reconfigured to serve the right MIME type, you need to generate a few mandatory icon sizes, you need a service worker even if you're not using it...
If you're using a big framework like Vite, this is somewhat easier, but then it's not a "simple html file" anymore and you spend just as much time fiddling with framework stuff.
If you're using a big framework like Vite, this is somewhat easier, but then it's not a "simple html file" anymore and you spend just as much time fiddling with framework stuff.