If you just want a simple informational landing page with maybe click through to other static pages to measure seo analytics then simple html/css pages will suffice. If you also want to capture email addresses then you need some sort of back end which you can build with something like rails or Laravel. But in that case I would go with an e-mail marketing platform like convertkit which has built in analytics. However it looks like you want the ability to build quick interactive prototype of your startup idea to validate the idea, in which case I would go with rails or Laravel which allow for quick prototyping and iterating on your idea. You could use htmx with both those frameworks to avoid a full blown JavaScript front end, but rails offers Hotwire and Laravel offers livewire which are packages that are integrated with the respective frameworks
You don't need a framework for a single page. That being said, using a framework is probably a good idea if you intend to be on a growth curve.
Pyramid is a framework written to scale from a small, single-file site to a site that has grown horizontally scaled across multiple servers. If I understand your needs correctly, Pyramid is probably worth a look, here's a link: https://trypyramid.com/
Pyramid is under active development, and it has a large, helpful, welcoming user community.
No you can write it in pure HTML and CSS. It’s really not that hard. Host your static files for free with GitHub pages, or it you want more formal deployments, pay $10/month for DigitalOcean. A custom domain can be had for $20/year.