Personally I would use htmx and roundtrips for all sorts of modification including data (such as reorganizing two rows in a table). But you'd also do that in an SPA, right? How do you prevent desyncs there? Also for e.g. sorting you'd need a server roundtrip anyway in the (likely) case where you use something like pagination or lazy loading.

For sending data, you would just have a reply that instructs HTMX to display a success message. On an SPA you'd have the same. With both, you can interact with the page while the data is being sent.

Of course, sometimes you want purely 'cosmetic' actions, such as an "add row" button that pops open some data entry fields. For something like that you should not use htmx itself, but instead basic vanilla JS or a simple library such as https://alpinejs.dev/ which complements htmx nicely for client-side stuff.