In both cases, you can force a whole-page refresh at short intervals by injecting a bit of Javascript code. Yes, I don't like that either 😉

Or with a more Javascript-heavy approach do targeted page updates. The "LED Animation Console" (DSL Code Editor/Compile) is an example of the latter, to disable/enable buttons.
https://github.com/arendst/Tasmota/blob/development/lib/libesp32/berry_animation/src/webui/animation_web_ui.be
Yeah, that's "a bit much" to handle a dialog.

It does not look like you can do much responding to the web_sensor callback, where the associated tasmota.web_send and tasmota.web_send_decimal calls are only for the specially formatted string, which is part of the standard refresh cycle (2.345 seconds intervals by default). Of course, it could be doable to have some Javascript thing watching details of that response to do something.

To be clear, I'm not declaring "that's it", instead only reporting details I've spotted.

Good Grief! My Belly buttons are really yelling for Help!

You can stop bellyaching now....

With AI help, I found a simple approach, only slightly kludgy, having the web_sensor callback respond with
tasmota.web_send(f"<img src='data:x,' style='display:none' onerror=\"let obj=eb('{sliderid}');if (obj) obj.{value=};this.remove();\">")
That "image" is guaranteed to fail loading (will not ask the server), hence the onerror will be executed. And the slider jumped.
Does not work with tasmota.web_send_decimal

That was for a slider, but should work as well for other types of DOM updates.