I've been considering htmx for some of our more complex forms with a lot of colocated information. One approach I have in mind is using a small frontend state management library like Alpine.js with the htmx alpine-morph plugin.
In short, if a required field in a different section, i.e. used for a "complex calculation" is changed by the user, then we'd pass that flag in part of the POST request to recalculate.