As for onslaught and rage cost, yes everyone knows that rage persists until you leave a zone and that you could keep onslaught up and ready for use if you keep at least 20 rage available.

It's annoying though, which is why it's a qol request and not something anyone thinks is manadatory for the job to function.

As for onslaught serving as a guage managment tool, I think it's clunky to have a movement action tied to the separate function of guage management just as how its clunky to tie a movement action to dps. A better design would be to either create a new guage managment tool/action or adjust upheaval to better function as one as dps and guage management pair better than movement and guage management or movement and dps.