I've not used Htmx, but a cursory browse of their docs gives https://htmx.org/extensions/multi-swap/ which seems to solve exactly this problem. And thinking about it, what makes it as difficult as you say? If you've a js-library on the client you control you can definitely send payloads that library could interpret to replace multiple locations as needed. And if the client doesn't have js turned on the fallback to full-page responses solves the problem by default.

Of course, I've not used Turbolinks, so I don't know what issues applied there.

Edit: I'm not saying htmx is the future either. I'd love to see how they handle offline-first (if at all) or intermittent network connectivity. Currently most SPAs are bad at that too...