To answer your question, the system is definitely not as flexible as I'd like it to be today. The eventual goal is for Reflame to support many of the most popular development toolchains out of the box, enough to make it possible to run alongside them in the same codebase, across many frameworks.
But we have to start somewhere, and create-react-app is where we've focused our attention so far. I want to work on supporting Vite-based React apps next, and where we go from there is pretty much entirely up to community demand.
There's also no technical limitation stopping Reflame from supporting frameworks outside of React. In fact, I'm pretty sure if you stuck with production mode live previews that do full browser refreshes on every change, you could just swap out the React initialization code in the entry point with a Vue/Solid/Angular version and it should work just fine (hit me up if you end up trying this and running into blockers, happy to look into what it'd take to unblock this use case). However, there's just a ton of work that needs to go into building a first-class HMR-style, state-preserving workflow for each framework, and until then I wouldn't be comfortable with advertising first-class support for them.
Unfortunately, I don't think we'll ever be able to support a fully custom webpack config that's not based on a popular preset like create-react-app. The surface area to cover is just too gigantic there.