I've used HTMX quite frequently recently for internal frontends (admin pages, reporting, etc.). It's quite good.

A pattern I've been using a lot is not changing the backend at all - each endpoint still returns the full page. Each hypermedia control makes liberal use of hx-select and hx-select-oob to update the elements that may have changed.

This isn't super efficient, but it provides some "locality of behavior" in that you can look at a hypermedia control and know what other elements might be changed by it. It also means the backend doesn't really need to "know" about HTMX.