Enrolling in Beta channel to prevent Tahoe updates

Sequoia 15.7.7 (OCLP) on iMac 2017 (A1418)

Since the trick of installing a system profile to block major version upgrades is no longer working, the only workaround to prevent Tahoe updates from showing up in Software Update is apparently to enrol in a Sequoia Beta channel. I've followed the instructions by signing up on the Apple Beta page with my Apple ID (https://beta.apple.com/en-GB/enroll-your-devices), and also tried to manually enable it in System Preferences using the command

open "x-apple.systempreferences:com.apple.Software-Update-Settings.extension?action=showBetaUpdates" 

but the beta channels don't show up and just says 'Off'. Tried logging out and back in to this Apple ID, rebooted etc. Any ideas please?