It's easy to be cynical and say that Loudspeaker 'doesn't work' if you've had an idea that was rejected, but the nature of Loudspeaker ideas are summarized and tracked frequently by senior leadership (C suite included).
The more people that submit their concerns, the higher up the list RTO becomes on a report. We need RTO to go to/stay at the top of those reports.
There's really no other way to let leadership know how badly RTO is affecting employees and their work. The C suite will automatically assume that everyone 'got used to' RTO if employee feedback dwindles (despite actual poor sentiment being the same).
Otherwise, you must be hoping that the C suite is reading this site and will change their outdated behavior (this site alone won't do it).