If you wanted to create a niche content site what platform are you building on in 2023?

Headless WordPress is pretty nice. I've also done some hybrid stuff in the past. You can load a small javascript app onto a WordPress page since all it really needs is a dom node to attach to. WordPress lets you load custom js bundles on a per-page basis so you can load your app only on the page that needs it. Then you can build your calculator/interactive examples with whatever library you are most familiar with (React, Preact, Solid, JQuery - if you want to go old school) and just interact with WP through the GraphQL or Rest APIs. Then WordPress can control the main parts of the website like the blog and the normal pages. All of the WordPress plugins will still work. You don't have to worry about SEO on the majority of the site since WP will handle everything but your little apps. You also don't have any of the complexity of going full headless. I've built a full headless WP with a Next.js frontend and, even with the headstart that Next gives you, there is still a lot to set up. TL;DR I'd say if you are trying to integrate a content site into an application then go headless WordPress. But if you just want a content site with some application-like features then embrace WordPress and add a few small js apps to the pages that you need.