> the most compelling reasons to use HTMX instead (for building dynamic/interactive web applications) are to reduce the amount of JavaScript sent to the client and make pages become interactive faster

Next.js and TanStack Start don't send any JavaScript to the client if you decide to server-render everything. The upside is that you can opt-in into client-side components, and have all the interactivity benefits of React. I understand using HTMX if you are building a backend in Go/Python/..., but if you are already using Node, I see no reason not to use TanStack Start instead.