> At that point, especially when you have many inputs like this, the amount of work one has to do with the HTMX JS API starts to look at lot like the script tag in your example

Not sure exactly what you're talking about w.r.t 'the amount of work one has to do with the HTMX JS API', but I've found that form validation with htmx and a backend server works really well with just a tiny bit of JS and a little care crafting the backend's validation error response: https://dev.to/yawaramin/handling-form-errors-in-htmx-3ncg