If you really want to fight RTO, then the most immediate and clearest way you can do so is this:
Do not submit your Voice+ survey.
Many of the higher level execs have probably minimum participation goals set in their performance goals. They care somewhat about a bad survey result. But not entering the survey at all directly targets their pay/bonuses. They might have thought that this performance goal was an easy "gimme" and when they see the low participation rate, they will freak out.
Outside of Germany, the survey is confidential, not anonymous. They will almost certainly know who did not fill out their survey. They will start demanding people fill out the survey not caring at all about the content.
If you are especially brave, you can then ask in writing if declining to enter the survey makes one "subject to discipline up to and including discharge, consistent with applicable laws" (use the same text as the nastygrams when they first moved to 3-days and then 4-days RTO).